Only very shallow women will care about this if they really like someone. Women are much more interested in someone who is caring and supportive - they aren't going to leave you for a well hung jerk if you're a nice, loving guy!
As others have said, learn to be great in bed in other ways, and even if it did matter (which it won't for many), you can find ways to keep her satisfied. Tip: The internet is full of advice.
That said, lots of girls down want a big guy anyway - it's not always comfortable (just as some men are small, some girls are small, too!), and some guys can be terrible in bed because they rely on a big penis and don't bother learning about women's bodies and what they enjoy.