I've seen a couple of fairly monstrous cocks in my time.
There was this incredibly cruisey washroom in the basement level of a passport office where I lived on the west coast. People would go down there regularly for understall action, and play at the urinals. Mostly it was understall, as it was more discreet and you could pretend nothing was up if someone walked in.
Anyway. There was a cyclist (courier?) who I would give handjobs to if he and I happened to be there at the same time. I would say he was 10 inches. Beautiful cock. There was another guy, a shorter Italian guy that always wore dark sunglasses, who was probably 10 as well, but was uncut and much thicker. I watched him get sucked off once, and gave him a handjob at another time.
There was also a guy that I went to school with, that would sit beside me in Algebra. He'd let his cock hang down his leg, and he'd move his leg back and forth until he started to get hard. Considering the tip seemed to only be 3 inches from his knee, I'd say he was at least 11 inches. He was curious, and I probably could have blown him, but he was a longer term project and our paths didn't cross that often after that class.