I would have agreed with the "size doesn't matter" group until recently when my wife and I met a black guy with an enormous dick (at least 10" long). It seems like it took him for ever to work that big thing in her, but once he did she screamed, bucked, and actually came over all his cock. She said it was the most intense orgasm of her life and the first time she had actually had an orgasm just from penetration. She is seeing him again.
Thats my 2 cents worth.