All the surveys seem to say similar things, but cocks are so diverse that there's really no way to make any kind of blanket statements about them. I have seen every possible kind of cock and none of them were alike.
Take mine for instance, 6.5" length, so I'm above average there. Above average "mushroom" head". And then a 4.5" girth (not very impressive there). I'm a shower, but don't much bigger when hard. People have told me my cock is big, small, average, and some failed to comment even when asked. I fucking HATE my lack of girth. But now I just brush it off and play all sides of the size spectrum for whatever fits the situation.
I have been with a guy who had 12 x 7. It was fucking hot, but I could barely get it in either hole, and neither of us could really service it like it needed to be. I've been with a guy who was like 3.5 x 4. His dick didn't turn me on, but HE did and would make me cum repeatedly every time. And I've seen everything in between - big that couldn't get hard, big attached to short skinny twink bottom boys (who won't even use it), small on big hunky hot muscle dudes, medium on boy next door with a heart of gold types etc etc etc...
Girth is really what throws everything off, because it's not the first thing you think of when you hear "his dick is 5". A guy could have an "average" cock, or so he thinks, but be bigger volume wise than the 7 or 8" guys he's trying to compare to in his head.