I'd agree with Tom, they want you hear you have whatever perfect measurements they dream of. Physique-oriented as most men are, they/we like to imagine that whoever we're playing with has that perfect hourglass figure and long, [insert fav. colour here] hair.

But why just imagine when you have someone roleplaying with you? You could not ask and just pretend, but when you are acting out fantasies with someone else, you can mostly depend on the other to play along. And it is that much better to hear from your playmate, right? That's the whole reason for roleplaying...

Sorry for the generalised answer, but, to be fair, it is quite a generalised question.