I myself prefer to work in 1024x1024, for less detailed heads, and 4096x4096 for more detailed heads.
because as we all know, the higher the res, the better artist you are, and the bigger your e-penor is.
(I see no reason to work in anything bigger than 128x128, and so i dont, except if i plan to render. No point wasting time on an image 4-16 times bigger than it is going to show up as. *shrug*)