C's, bordering on D's are perfect
However, if the girl has a pretty face and a nicely shaped body, the size of the boobs are not as critical. When it comes to porn, it seems that girls compensate for below average looks with enormous breasts. This happens to some extent with Asian porn but even more so with western porn. If the girl isn't pretty enough, she goes for huge implants because it's the only way she'll be noticed and have any success in the biz