While I agree that the whole package makes the woman, I prefer smaller women because I'm not that big myself. Last time I dated what is now considered a "medium" sized woman (what I would consider large) I felt like I was crawling all over one of those air mattresses you float around the pool on....
I have a rule of thumb: if she can't fit into my jeans, her booty's too big for me! (I take a 32" waist btw).