Thought this might be interesting. I found this site some time ago in my never-ending quest for penis information. Groupies post information about sex with rock stars, including their penis size. Now, I'm sure some of them are bogus but I think that most of them are pretty factual. And, I believe most of them are pretty accurate on estimating penis size. IIRC, the largest reported was 10", which is pretty much the upper end of the curve.
I know that many of you have stated that most girls can't accurately judge penis size. But I believe girls lots of experience can get pretty close. And I know for a fact that many of these groupies are extremely promiscuous ( to put it delicately.) You've probably heard stories about what they will do just to get backstage. A famous true story, a couple of groupies in the 60's went around making penis molds of rock stars, and Jimi Hendrix had the largest. And, I don't think that those girls' motive was strictly acedemic ;-)
Anyway, it's a fun read, take with a few grains of salt but I think much of it is probably true. One interesting thing to me, more than one commented about guys with large penises, but they couldn't keep it up. They hate a "wet noodle", probably caused by drugs and/or booze.
Here's the link: