I remember that in Darkfall most "serious" pvper took dark elves just because they had the smallest hitbox. Hope this kind of min maxing is somehow discouraged.
In DF2010 they actually made all the models the same size because of this. I think you can have different sized models if you put a lot of thought into balance. Like the smaller your hitbox the less damage you do. They planned that with Darkfall originally (the cat race would be big but could run on 4 legs)
I think overall its too much hassle and messing around for too little payoff. Just leave all the models the same size and let us have as ugly a face as possible. Broken noses, scars, inbred jaw etc. Even small changes in hitbox will result in people re-rolling for the advantage