Ha, I'm trying to remember where I learned that as well. I know we covered it in drafting where we learned an 8.5x11 A paper is half a sheet of 11x17 B paper which is half a sheet of 17x22 C paper, and so on. But I thought I knew the size of A paper long before that, and that it was common knowledge, though I can't think of where or why I would have needed to know. Then again I also know that legal paper is 8.5x14 even though I have never had to use it.

Grade school for me - teachers would say "8.5x11" instead of "letter size" or even just "printer paper." I don't know why they did it, and I assume it's for the same reason that I say it too. It's probably what their teachers said to them!