I've always gotten the feeling that size represents roughly how tall or long a creature is, slightly larger than feet.
Someone should go through the raws and catagorize the sizes so we know what we are dealing with.
Ok.
None: vermin, all small fish
2: hoary marmot, fox, raccoon, groundhog, stingray
3: cat, fire imp, carp, all gnomes, rhesus macaque, all gibbons, milkfish, cod, bluefish, tigerfish, sea lamprey, spiny dogfish
4: ratman, batman, antman, gremlin, cave swallowman, large rat, siamang, naked mole dog
5: kobold, dog, all wolves, mountain goat, snailman, slugman, leechman, frogman, olmman, longnose gar, pike, deer, warthog, cheetah, mandrill, chimpanzee, bonobo
6: dwarf, goblin, harpy, troglodyte, leopard, strangler, opah, great barracuda, giant mole, frill shark, angel shark, coelacanth
7: human, elf, donkey, cougar, jaguar, fire man, magma man, iron man, mud man, tigerman, lizardman, snakeman, beak dog, grimeling, foul blendec, orangutan, merperson, satyr, conger eel, elk, spotted wobbegong, all reef sharks
8: werewolf, black bear, mule, lion, tiger, gorilla, giant cheetah, nightwing, giant rat, giant cave swallow, common skate
9: horse, cow, grizzly bear, troll, ogre, unicorn, minotaur, sasquatch, blizzard man, muskox, giant grouper, halibut, alligator, giant leopard, nurse shark, all mako sharks
10: giant cave spider, giant eagle, all camels, all crocodiles, walrus, sturgeon, polar bear, giant jaguar, giant desert scorpion, giant toad, giant olm, giant bat
11: spirit of fire, giant lion, giant tiger, treant, marlin, bluefin tuna, bull shark
12: frog demon, tentacle demon, ettin, wagon, hippo, ocean sunfish, swordfish, blue shark, hammerhead shark, manta ray
13: great white shark, tiger shark
16: demon, giant, cyclops, elephant, whale, sea serpent, sea monster
18: basking shark
20: dragon, titan, bronze colossus, hydra, whale shark