As long as its somewhere between 6-8 inches its fine.
I've never had a small one or a big one, just average ones, and even one uncut one (foreskin is fun to play with). I'm more concerned whether its trimmed or shaven, if its too hairy it just makes you want to laugh. A little Man-scaping never hurts gentleman.
The real factor is the penis' heath, if a guy has poor circulation or is slightly overweight, the penis might not be able to get as hard or as full as it could, size isn't so much as important as how fit and hard it is.
Just be able to deliver a healthy creamy load and she should be satisfied.