Penis size is not the only thing girls want for satisfaction and there are many, many more things.
If you have a 15 inch penis and a very bad personality, I don't think many women would want to be with you. And if you want to be with the girls who'd still want to be with you for your huge penis, then keep in mind that they're ONLY with you for your penis. Nothing else.
Yours is a good size, and I'm sure girls would be more than satisfied with it. But make sure you develop your other MUCH MORE important aspects of personality as well.
Cheers!