Well, I mean, you may have some difficulties reaching the right spot and/or stimulating a girl in just the right way during vaginal sex.
But here's a news flash! Most girls don't orgasm from vaginal penetration alone, and even the ones who can might not be able to do it every time, or might not be able to do it without clitoral stimulation as well. Most girls are more likely to orgasm through fingering or oral than they are from sex (and particularly clitoral stimulation). So, I don't think any guy can use the excuse of his penis size for not being able to bring a girl to orgasm. Explore with your partner what kind of stimulation she needs to reach orgasm, and then stimulate her in that way using your fingers, your tongue, or toys. This isn't to say that you can't also have vaginal sex with her, but certainly don't make it the only thing you do with her and don't think of it as the only thing you should be working on to bring her to orgasm.
Don't worry about the size of your tool. Just learn how to use it. And, most important of all is learn how to use your fingers and tongue. Just get her to cream (not scream as another answerer said) before you stick your d*ck in. Also learn to grind your pelvis against her pelvis. This puts pressure on her clit and we all know that is where the fun comes in.