I know somebody where this was the case. She claimed he was 9", but Im willing to bet it was 7". Anyway, she said that sex was always problematical. It just wasnt worth it. Whats the point if pain is involved everytime? And they broke up for THAT reason.
Then she found some other guy, I guess "average" size, and she said it was perfect. Anytime, anywhere, and no "prep" work beforehand to "loosen up".
So I guess size does matter when it comes to sexual compatability.