Penis too big?

Yes, there is such thing a penis that is too big and contrary to popular belief, woman don't really like big penises! I don't know if there's an average depth of the vagina, but I know most "normal" woman are "short" inside. I am and my husband with his "tiny" penis hurts me sometimes too, depending on the position. I can't imagine having sex with a larger guy, it wouldn't be pleasant at all. You can't really do any damage to your girlfriends body, but if she's feeling pain, she's not having a good time. So, what I would recommend to you is to try different positions to find the ones that she will feel comfortable with.

ps. when I say "normal" woman is to differenciate from porn stars and prostitutes that do exercises to lenghten they vaginal canal to accomodate larger penises!