I haven't compared with many guys in person, but I have via Facebook chat or text message. About half have actually traded boner pics with me, most of them gay though two of them entirely straight (one shortly after got married and now has a kid) and one who's currently desperately trying to pretend he's straight.
I tend to be bigger or about the same size (of the ones with pics, I have one good friend who's about the same length but not quite as thick, and one who's almost two inches shorter but almost exactly the same girth and weirdly similar shape; I have two other friends who claim to be around the same length and one who's half an inch longer, but I've yet to see photographic proof). There's also one guy who has a pic of his thoroughly average cock on an old porn-reblog-Tumblr (and it's clearly his, since the random crap in the background EXACTLY matches the Facebook profile pic he had at the time), and, when I asked if it was his, he claimed to have a much bigger one, then sent me what was obviously a set of photos screencapped from Grindr.
My favorite story, though, doesn't even include whipping it out... when I was 16, I was working one of my very first jobs, and one other employee--a college guy--and I were closing up one night. There was no one in the store, so we had the TV on, and there was a "Friends" rerun, which started with Monica saying "They say it's the distance between a guy's thumb and his pointer finger" and Ross asking, self-consciously, if he can use either thumb. Matt, who was a cocky guy but had mostly small features (fingers included), stretched out his fingers and braggishly said, "Yeah, that looks about right." So 16-year-old me holds up my hand next to him, and, since I've been described as having E.T. fingers, my finger-span was already bigger. Then I pulled back my double-jointed thumb. I don't think Matt talked to me again the rest of the night.