and here we have Tabby stating the false and obvious

1, bodySize is NOT unimplemented, it is fully functional (it's how you can change the character size, tho prey only, as you mentioned: animations) xD
2, not entirely false (and i can see why you'd think that) but recruitAsCrew has no effect on body size, it is possible to recruit a large npc (i know bc i did it)
Now tell us how the bodySize parameter in bytes file affects "the character size, tho prey only".
Otherwise I have to call
"And here we have FurryLion stating the false"
I've tried to change the prey body size by modifying the bodySize parameter in bytes file in the past, I've tried it again now:
Took bytes files of two custom characters that don't resemble any existing prey in the game.
Modified bodySize parameter in one file to 1, in the other to 2. (Enum, thus 32-bit integer, little endian)
Created prey characters in a clean pail file, one for each rarity for each bites file to increase a probability of being chosen by spawner.
Created custom prey spawner(s) for that (sub)species (not forcing characters) with equal SizeChance values on some isolated peninsula.
Tested the game with all other mods disabled to avoid any interference.
Results: Original game prey spawners spawn both characters in medium size, ignoring the size parameter in bytes file.
Custom spawner(s) spawn both characters in ALL sizes, ignoring the size parameter in bytes file too.
Now just for sure set spawn_unaltered to true in characters definition. I guess that the spawner would not use the character if it does not meet any parameter value. Nope, no change for both original and custom spawners, happily ignoring the bodySize parameter.
Since the recruitAsCrew seems like the only function actually using that parameter in bytes file (not tested that before), I wonder why no NPC in the game has that utilized, not even new ones, like Arktis and Tilli on Saardan.