7" or so is about right, and this is one of the rare occasions when I recommend a knife with a hollow ground edge (as Amanda pictured), given that most of the work is slicing. However, I usually also keep a heavy, very sharp chefs knife handy for cutting through and around ball joints; legs, wings etc.

Should not be too long or it gets unwieldy. I like one that's 6 to 7 inches long with a blade that has a little flexibility.