Well ideally they would be pretty ample. I think the amount of cleavage she has is more important. I can't give you a certian cup size, as all breasts are different. A full pair of D's should be no problem to breast fuck. I bet with the right shaped A's you could do it as well. A nice As for cock size... it has to be hard, that is all that matters.

If you are into breasts, you should give it a try. Have her hold her breasts together as you work your cock between them. I like it every now and again.