They just run a size large. I am size L, but get M in the long tail tee. You could also buy a few different sizes and return what you don't like. The tees are really great.
I bought one and it works good for me, cause I was just looking for a shooter. They let me check out a few and some were in pretty bad shape, so I wouldn't just pick the first one they give you. It definitely has quite a few rounds down range. Fully broken in you might say. Came with two mags...