I don't think size really matters, because in reality, the bigger the dick the more it'll hurt the chick.
Chicks say that they want big dicks because, in reality, it's a psychological issue where they feel they are in control. It seems it's mostly for bragging rights. "I own this big guy and I get to brag to my girlfriends!" seems to be the mentality.
But really, there's not much to it to fucking. You stick it in, you swirl it around while it's inside by vibrating or moving your ass, you pull it half out and tease her.. If that doesn't work, you can press down on her tummy area a bit, which, depending on the position, can enhance the dicking. While she's in the cowgirl position suddenly massage her ass hole. It'll feel good and make more into it. You have to do it fast though, otherwise she'll know whatyou're doing and be all,"You are NOT touching my ass hole."
Even if you have a small dick there should be some way to work around it. The biggest problem with guys with small penis is that they have insecurity issues, that's their biggest obstacle. Length is not the issue here, thickness is.
If you get past it, use your imagination (fucking is not THAT deep), you should be able to win gold. In the end, what really matters is whether you made her cream or not.