The mean size of the human penis is 5.1 - 5.9 inches in length...studies that rely on self-reporting showing a larger number
A penis under 3 inches, but otherwise normally formed, is referred to as a micropenis. (Wikipedia, right?)
So tell your self-conscious amigo that his dick is just at the long end of average. If that's not sufficient then most men can't satisfy a woman, which is nonsense. His (obvious) task is to be a better-than-average lover.
As to mostly men responding, two thoughts:
First, the women on this site are in the business of making men feel good about themselves sexually. Asking them to rate men by cock size is a non-starter. What you'll get is (a) "yes baby that is plenty big enough," and (2) "interestingly, that's exactly the size I prefer!" both of which may be true but both of which have now been taken
Secondly, it is men (remember) who are self-conscious about penis dimensions. The standard defense (at least on this thread) is to downplay one's own penis, claiming in effect to be possessors of micropenises.
My dick is larger than the mean, closer to 8 inches (self reported!) I have to tell you, women seem to appreciate that extra 2 inches. True, occasionally one encounters a willowy lass who says "ouch," but that's rare (some thin girls grind down to get it all!) and, as someone mentioned, a woman with some meat on her butt loves to get a deep fuck doggie style.
That said, your friend (alas, unlike the short-dicked respondents) has plenty to work with. Statistically speaking.