The first time I was measured or even realized I was big was when the girl I lost my virginity to decided she had to measure me after a conversation she had with a friend of my ex. I guess my ex, who I had only fooled around with, had told her friend that I was well endowed; the friend then asked my girlfriend for tips on dealing with my size. My girlfriend and I were both virgins when we started going out, and she had never fooled around with anyone at all before, so she didn't realize that I was big. She told me about the conversation and asked if she could measure me because she was now curious, of course I said yes. I don't think I had really even thought of it before then.
The girlfriend I had after that was also a virgin, but she had a friend who was a size queen and kept bugging her about me; at first she didn't want to ask me if she could measure me, so she just measured both of her hands together and told her friend that I was that size. She was pretty close, that was about 7 and 3/4" and I'm a little over 8". When she did ask, the first thing she did was tell me she'd have to tell her friend she underestimated :-\