I dunno, bigbull. I used to think that my length was comparable to that of many porn stars, but eventually realized that in many cases I was under-estimating the size of these guys.
When I first started looking at porn, I thought most of the guys seemed average to slightly above average--either about the same size as me or just a little bit bigger. Of course, there were some exceptions: Jeff Stryker, Ken Ryker, and Matt Powers come to mind, but I perceived myself as being big enough to do porn if I wanted to, although I'd never be one of the big stars.
Since I joined LPSG, I've been paying more attention to size, and looking for clues as to how big a guy really is. It was not always easy. I certainly can't estimate a guy's length to the precise inch, but I realized that if a guy's dick reaches past his belly button, it's probably big. If he can fuck his own ass, it's big. If he's squatting down and his dick hits the ground, it's big. If it lies in the grass like a snake, it's REALLY BIG.
So I was chagrined to discover that many of the guys whom I perceived as "average" (i.e., about the same size as me) were often bigger than I thought. I think it's logical to assume that if a guy looks huge, then he's probably several inches longer than me.
Oh, to put his all in perspective, my cock is 6" long by about 5" circumference.
So I'm guessing that the "average" porn star is a good 7"-8" (or more) and that the guys who look really big (e.g., Matt Hughs), may very well be 9"-10" or more.
I think the important point is that if none of us can reliably tell if a guy is 8" or 10", does it really matter?