Hey guys, I have no idea where to find this out so I figured this was the best place to ask.

I'm doing a paratrooper outfit for a halloween party and I just about have everything but one thing. Leggings. Im lookin all over sites for a pair, but size wise I have no idea. I see things like, 3r,1r, and so on, but I have no idea what that means. Does anyone know what "r" size I would have to buy to fit a size 13 boot?