It was when I was in 7th grade, and my older sisters' friend who had sort of a reputation, made a move on mewhen my sister was out of the room. I was waaay excited of course and was showing down my leg-this is back when I wore my pants not so baggy-'94, and she saw it right away and said "Oh my god, thats not you!" I of course insisted it was. This went back and forth for a minute or two, she thought I had stuffed a roll of socks in my pant leg, until finally she decided to go for it, and grabbed my cock from outside of my pants- now I have big hands and never was really sure about my girth, I sort of had an impression, but as soon as she grabbed my shaft-at the widest part, no less I knew-she couldnt get her hand around it. She thought it was big a minute ago, but I wasnt even at full yet, and when I started growing she said"Oh my god!' and covered her face with hands, embarassed, and laid back on the bed with her eyes closed, and waited. Of course I had no game then, and left her there after some embarrasing exchange or another and jerked like it was my job-I came sooo fucking much that time, and of course I had a memory that would supply years of jerking material.
It confirmed for me when I accepted a condom from someone passing them out on the street, figuring I should know at least How to put them on, and couldnt get it halfway down my shaft, and that with great effort, it just wasnt wide enough...it was so tight that it hurt and wouldnt go anymore.