Most women can't orgasm by stimulation of the vaginal wall alone, and for them, sex is more in the mind than the body(for the most part). Just remember that the clit is where it's at. Even if your dong is huge, you usually have to stimulate the clitoris anyway for the woman to reach orgasm. Also, remeber, that women make love with their entire bodies and not just the genitals and normally thought of erogenous zones, as men usually think of it, so take that into consideration. So, regardless of your size, your penis alone is not usually not gonna do the job by itself.

This is just what I (a man) think, so if anyone of the female persuasion would like to correct me, go ahead.