Try checking one of the 3D models that users have made for external modelling programs. The sizes should be proportional, if the user did a good job.
Alternatively, create a mod ingame in which each bodypart is a tiny bit bigger than the original. Then, open the mod in a text editing application, and the sizes of the bodyparts should be relatively close to the original values. (as far as I know, classic/default.tbm and mods in which default values are not changed do not show the sizes in the .tbm file)