Body sizes are in the creature files, not the entity files. Just ctrl+f for [BODY_SIZE: to see the various caste values.
Most leaders are 1,000,000, except Underlords who are 1,250,000 but slower than other royals, and sacred flying buffalo, who are 5,000,000(so don't go antagonising them unless you've got a cunning plan). For cave ponies, slippies are 150,000, gorlaks are 200,000 and dralthas are 450,000, bringing the average right up.
Apparently this scale is based on cubic centimetres, not grams like I thought it was when I started, but the two should be roughly equivalent. I based it on a comparison of pony and zebra sizes, 250 Kg being the lowest weight for a full-grown zebra I could find. It also works well with buffalo, since they're about four times the size of ponies in the show, and weight about a ton in real life. So ponies are about four times the size of dwarves, or at least two and half in the case of slippies, which is why unarmed ponies can be a bit brutal. I've had an unarmed pony take down an elephant. In the initial release, they weighed 450 kilos. Someone mentioned it as being like Kobold Camp, but in reverse.
Reelyanoob, I wish you hadn't told me that. When I have the choice of doing something fun and high risk, or something utterly boring that will make me invincible, I always go with boring.