A penis of less than five inches. But most girls are willing to deal with a small penis if there is a caring and sweet guy attached to it. There are other things you can do in the bedroom with a girl, and LOTS of things to do outside the bedroom that mean a lot more to us. Yes there are some shallow girls demanding 7+ inch penises but most of them aren't worth the time of day anyway. The whole size thing is overrated.
Would I rather be in a relationship with a sweeheart with a small penis, or a total jerk with a big one? I'll take a small sweetheart any day.