Uh, im not sure what your asking, you mean like making aboms 3rd arm the same size?
without alot of calculations, its next to imposible to get the exact same volume of body parts..
however you can use war3modeleditor (in tools here)
to grab a part, and scale/shrink it untill its aproximately the right size.. but itl probably never be exact
oh ant w00t 1,000th Post!@!