Growing up it was obvious that people were all different sizes, but I didn't really think much of it. I also had larger feet than a lot of other people, and was taller, so I just figured that I was growing faster and that the other kids would catch up eventually.
It wasn't until I was exposed to people discussing the unreasonable size of men in porn. I initially thought "yeah, a few of them are pretty big" with people like Mandingo, but I didn't realize that they meant most men in porn.
I distinctly remember a conversation where someone was trying to explain Peter North to me and said that he had a really huge shlong. "It's like "ten or eleven inches..." they explained.
I thought to myself that eleven inches was pretty big indeed. Then I saw him, and didn't understand what they were talking about because it didn't look all that big to me. That was the moment where I first started to really think about size in an objective way.