Depends on the partner. I had sex with a 9,5 inches (and pretty large) man, it was at first very painful, my ass is rather tight... and I think it hurted him too. ^^ But he was very respectful, very sweet, and with persistence, it was great. I restrained myself from cumming to make the pleasure last. However, he told me he had involuntarily bleed some of his partners.
It is difficult to tell what do we feel exactly. As far as I'm concerned, I'm less excited by the penis itself than the man who pounds me. In the case I described, I was mostly excited by the real man he was, his virility, his strength, his powerful body, his gentleman manners and the fact he did not talk very much about his huge cock. Without false modesty, he knew he was blessed and was very proud of it but he didn't use his cock as an argument of seduction.
In comparison, I had a very boring experience with a 9 inches man. He was much more beautiful but a bit brutal, arrogant, selfish. I didn't get excited and he was upset. Of course bigger is generally better, but few extra inches in the ass would not have changed anything in that case.