For what purpose are we designing the chicken?
If we're designing it as a food animal, I'd maximize the meat area and try to minimize the rest. I'd reduce the amount of body fat naturally carried. If possible, I'd take a page from "Top Secret"[1] and make the bird perform photosynthesis.
If we're designing a good prey, I'd make it smaller and give it better wings for flight.
[1] http://www.amazon.com/Top-Secret-John-Reynolds-Gardiner/prod...