Question for guys only? 10 pts!?

Okay. Me and a friend are going to have a sleepover (we're both 13 yr old boys) and I want to compare penis sizes with him. The thing is, i don't know how to ask him about this. We will both be alone in the basement in sleeping bags, so there won't be any interruptions. We always change out of our clothes into boxers or something else comfy to sleep in, and I usually take the bathroom to change, and he changes in the basement room while I am in the bathroom, and I come out when he says he's done. Would it work for me to just start changing without going in the bathroom and then go from there? Please help. The person who's suggestion works will get best answer.