Under the official rules, creature building begins with Hit Dice of different monster types. Hit Dice are roughly equivalent to levels and creature types to classes - each HD gives hit points, attack bonus, save bonuses, skill points and feats, in different progressions by type. There are also major features specific to the individual monster listing - ability scores, natural armor, movement rates, attack forms, communication forms, senses, bonus feats, unique abilities, etc.
Of course players can get along without noticing this because 1-HD creatures with class levels (including standard PCs) ignore their creature Hit Die and everything that it would bring. But they do also get ability adjustments, size and other features by race.
Almost as an afterthought, creatures usually have some kind of physical description. In the paid-for Monster Manual you got at least one illustration per main monster entry, and the later publications more routinely stated a typical height and weight, but in the SRD there is often no more than a line or two of description, sometimes very vague, and a size category.
I, of course, am not happy with this. My basic problem is that physical description has little relation to stats. Here's my list of gripes.
There's no direct connection between size or physique and Strength in the official rules. Of course larger and brawnier creature types tend to have higher listed Strength scores or adjustments, but this doesn't seem consistent. Looking at some animals and the human comparator for quick examples. Hawk, weight up to about 5 pounds (from Wikipedia, with wingspans under the SRD's listed 6ft), Str 6. Cat, weight 8-10 pounds (from web search, not listed in SRD), Str 3. Badger, weight 25-35 pounds, Str 8. Dog, weight 20-50 pounds, Str 13. Baboon, weight up to 90 pounds, Str 15. Monitor lizard, weight up to 150 pounds (assuming that at Medium size the designers were really thinking of the Komodo dragon, up to 10ft long, rather than a common mainland type such as the Bengal monitor, which does fit the SRD's length listing of 3 to 5 feet and weighs up to about 15 pounds), Str 17. Human, weight average 140 pounds (female) to 175 pounds (male), Str 10-11. Black bear, weight (from Wikipedia, not given in SRD) typically 150-300 pounds, Str 19. Light horse, weight (not given in SRD but size Large so at least 500 pounds, and from internet search around 800-1200 pounds for modern breeds mentioned in SRD), Str 14.
It tends to break down even more when dealing with more individualised characters who can have ability scores, sizes and weights that are not average for their type. For example a half-orc (max generated weight 438 pounds) can have 20 Str, which is basically the same as an average ogre (Str 21) with listed weight 600-650 pounds. A halfling (max generated weight 38 pounds) can have 16 Str, six more than an average human weighing three or four times as much. And there isn't even anything in the rules to say that these strongest examples of their race must be among the biggest. The Player's Handbook does suggest you might want to think about ability scores if choosing height and weight, but then it goes right on to offer an alternative method of random generation of height and weight, without reference to ability scores. The random method alone is given as the determining rule in the SRD, so your 20 Str half-orc is as likely to weigh 114 pounds as 438.
Now, I have particular difficulties with hit points in the official rules, because they don't differentiate between the bodily capacity survive injury and the learned or otherwise gained ability to avoid injury. There's a fuller discussion of that in my hit points explanation. But here let me just mention two issues: the relationship between size and hit points is rather loose when comparing different monsters; and the relationship between size and hit points is nonexistent when comparing standard PCs and similar characters.
Broadly speaking, larger monsters do have more hit points. Many monsters advance by adding HD, and the large majority of those go up into a larger size category when they pass a certain HD threshold. However, the HD thresholds, vary widely from monster to monster, and so do the relationships between the size category and the HD in the basic, un-advanced listing. In many cases monsters have unnatural features which may account for their hit points, or they have intelligence and lifespans which suggest they may have additional non-bodily hit points equivalent to those gained by characters with class levels. But even looking at mindless and mundane Vermin, there are inconsistencies: monstrous centipedes have 1/2 HD at size Small, and do not have 3 until size Large. But giant fire beetles have up to 3 HD and are always Small. Possibly that is an issue with assigning size categories based on length, but HD more on weight or bulk. So then looking at Animals, horses (size Large and quite robustly built) have 3 HD, or 3-4 if trained for combat, which is no more than many Medium animals. There are even Medium animals with 5, or up to 6 HD (mainly carnivores, I should acknowledge, but I don't see the justification for their being so much tougher size for size than a specially-bred and trained war beast). Actually, talking of carnivores, why are cheetahs (built for speed), and a number of other predatory mammals, always Medium up to their maximum 5HD whereas wolverines (known for their powerful physique and ferocity), and wolves, are Medium only up to 3HD and have to be Large if advanced to 4 HD or more?
Looking at another angle, any member of a 1-HD character race (those whose creature entry is for a level-1 character: dwarf, duergar, elf, drow, half-elf, githyanki, githzerai, gnome, svirfneblin, goblin, halfling, hobgoblin, kobold, merfolk, orc, half-orc, aasimar, tiefling and optionally pixie) gets hit points purely according to class (from 2 for a commoner or wizard to 6 for a barbarian), whether they be an average 30-pound female halfling or an average 241-pound male half-orc. There is a Consitution adjustment, but average members of those (and most other) races have a modifier of zero, and there is in any case even less connection between Constitution and size than between Strength and size. If you are following the SRD and rolling height and weight randomly, you could have your hp 6 halfling barbarian slightly lighter at 27 pounds, and your hp 2 half-orc commoner as heavy as 438. Bring in Constitution variability and the halfling could have up to 10hp (even 16hp if a PC or elite NPC), and the half-orc just 1.
To be fair, there are also some sensible patterns to this: creatures with hard or tough exteriors get better natural armor than those with mere skin, and larger creatures more than smaller. Mostly. But even for creatures with reasonably naturalistic physical forms and moderate Challenge Ratings there are numbers that don't make much sense (for example: giants, scaly humanoids like kuo-toa or troglodytes, monstrous scorpions and some other insectoid creatures, octopuses, vipers).
And there are many cases, especially at higher Challenge Ratings, where clearly the logic has run from the desired CR and combat role of the creature to a desired Armor Class, and when size and Dex modifiers didn't add up to it, natural armor was simply used as an arbitrary adjustment. Sometimes the description includes physical protection accordingly - I'm not arguing with dragons or purple worms. But there are human-animal creatures like the lamia, sphinxes, lammasu or drider, which have better natural armor than either humanoids or the relevant animals of equivalent size. The carrion crawler, which appears much like a ten-foot caterpillar, has better natural armor than a much larger and famously tough-skinned creature like a Huge shark. There are plenty of supernaturally-powerful creatures with humanoid forms, such as undead, outsiders, hags and so forth, that have arbitrarily high natural armor bonuses too. The nightwalker, with +22 at size Huge, and the pit fiend, with +23 at a scaly Large, are perhaps the extreme examples, though the succubus with +9 at Medium has a more obvious lack of justification.
My main issue here is that PCs and NPCs have fixed movement rates by race or creature type, despite having wide variation from their racial averages in height, weight, build, Strength, Dexterity and other physical parameters. If an average elf or human moves 30ft per round and an average dwarf or halfling moves 20, that is clearly supposed to be a function of stride length and suchlike. So why give the same base speed to every elf, human and half-orc from 4' 7" to 6' 10"? And whether they are a strong and agile person or an enfeebled couch potato? But the two inches difference to a 4' 5" dwarf drops movement speed by a third? I'm not happy with it.
Even assuming you have the core rulebooks, not just the SRD, you have relatively little information about the physical anatomy of many monsters. If you're lucky you will get: a picture giving a good overview of the body shape and clues to the composition and physiology; a line or two of description that match the picture and give helpful information; a couple of measurements (such as length and height at the shoulder, or length and wingspan) and a weight (that are believable together) for an average specimen, and a size category that fits the other figures. Too much of the time key information such as length or weight is missing, the description is really vague or contradicts the illustration, and/or the numbers seem inconsistent (the pit fiend's height and weight figures imply the same proportions as a human of 6 foot tall and 100 pounds, for example, which is clearly not what is illustrated; the size category for the base manta ray is even contradicted in its own advancement line, which doesn't seem to have been picked up in errata). One of my pet peeves is that very thin narrow creatures tend to be size-categorised by overall length, putting them in a category with creatures much bulkier and heavier.
The official rules assign various features to creature types and races with apparently no thought as to whether they are physical, mental, cultural or what. There is no guidance on what should happen if someone is born to parents of one race or creature type but brought up by another. For example, elves get bonus proficiency with swords and bows, because these arts are esteemed in elven culture. Half-elves (and other races) do not, even if brought up by elves. But half-elves do get Elven as an automatic language, even if they were brought up by humans and have never met their elf parent (likewise half-orcs and Orcish). And don't get me started on racial alignment tendencies or on the concept of 'savage humanoids'. I mean sure, you could change these for your individual campaign, but it bugs me that the rulebooks make no distinction between genetics and culture. In fact it bugs me that they assume that each race has its own culture and language to begin with - why should there not be cultures with multiple races, and races spread across multiple cultures? And why should the boundaries between them be sharp rather than fuzzy?
I think there absolutely must always be: at least a length or other primary linear measurement, some indication of at least a second dimension or relative bulk, and a weight. (Ideally I'd be detailing relative lengths and thicknesses of different body parts and calculating volume and mass from there, but to be fair that is a big job.) There should be an idea of variability around the quoted typical or average figure. There should also be some indications of physiology or anatomy, especially if it is fantastical or unusual. I'd like to know what you will see when you cut a creature open (face it, adventurers often do). And of course there should be a detailed description of creatures that have non-standard shapes. (My resources don't stretch to getting them all professionally illustrated.)
Most crucially, the stats should match the description. Larger, more muscular creatures should be stronger; larger, more robust creatures should have more hit points (body points); creatures with tougher hides should have better natural armour. The empirical evidence is actually that being bigger with a longer stride makes only a smallish marginal difference to speed, but body shape and other adaptations do make a difference.
Although in a number of areas (such as hit points) I'm separating the contribution of body type from the contribution of experience levels, wherever natural stats and experience contribute to the same thing, they should add up, like levels from different classes (and indeed like creature HD and class levels for classed monsters with more than 1 HD). Nowhere, not even for PC races, will natural stats be ignored and replaced by stats from a learned character class.
Natural stats include those built into a creature's body, and those naturally part of the way its mind works. They do not include things a creature has been taught, or learnt from experience. I will have separate classes for a creature's body type, for its mind, and for experience levels. Each class will include only features and bonuses appropriate to that kind of capability. Although body and mind will normally go together for natural creatures, skills, proficiencies and other cultural achievements will be learnable by creatures of any race or type (as long as they are appropriately intelligent, and naturally capable of the activity in question, in terms of having hands, organs of speech, or whatever). Even body and mind can be separated, when special cases such as shape-changing magic, possession or what have you come into play - this is why I make sure to keep them distinct in my experimental rules.