I have seen many butt or booty cakes on this site.
Some made with heart shape pans
But I can not figure out what ball or pan sizes is being used for the butt.
A ball pan seems too big.

I have only used the heart shaped pans

It all depends on how big or small you want your cake to be.

I do mine by stacking two layers of 1/4 sheet cakes and free-hand carving them. Not complicating at all.
If you must use a pan to bake "parts", i would think easter egg forms should do.
good luck...

I've only done one, but it came together a lot easire than I'd thought it might. I used a single layer 13x9, and then the two halves of the ball pan. I set the halves on the 13x9 (after slicing a little off in the middle for the halves to fit together better), and carved the sides of the 13x9 to give a little definintion for the waist..

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1310847