I can only recall two.

The first happened during high school, freshman year. A friend of mine was boasting that his dick was as long as our school lockers are wide, which in respect, isn't anything that spectacular but it was one hell of a comparison to make. We talked on the phone occasionally and in one conversation he mentioned measuring... I think he said 6.5. Something like that. I had an old tape measurer from my mom's sewing machine and wound up a little over 7. For some strange reason, I still had the anxiety that it was smaller than average -- maybe 'cause with respect to my build, I'm smaller than the average male.

The second: A college girlfriend and I were making out pretty passionately. My dick naturally responded and bulged out in my jeans; she remarked about how big it was. I assumed it hadn't grown until, at her suggestion, I measured it to its current size. Well, it was 9 then, and I fluctuate between that and 9.5" so no big deal...