The two gentlemen before me pretty much called it.
But also, remember that not all women can orgasm from intercourse alone. The majority can't. So for those women, it's not going to matter how big or small the penis is, the orgasm still won't happen.
And for the women who can cum from sex, they're going to be able to get off pretty much no matter what size the dick is, so it doesn't really matter for them, either.
So guys need to stop worrying so much about their size and focus more on technique (penis, mouth, and fingers) if they want to make a girl cum. And I do realize that this is sometimes easier said than done.
But if all else fails, break out a vibrator.