1) When two boys pull out their penises to see which one is bigger. Usually done by boys between 8 and 13. Anyone caught doing this after the age of 13 can be considered gay!
2) An excuse for a guy to look at someone elses cock at a urinal or in a open shower.
1. Hey Justin, lets go to my room and compare sizes.
2. I'm not gay, I was just comparing sizes.