Help with American Apparel sizing?

I won some american apparel shirts on ebay and now I have to puck the sizes, the shirts are the thermal long sleeve henley and the regular long sleeve thermal shirts. I don't know what size I would wear though, I;ve checked their store reviews and people have said different things about the sizes. I'm about 165 lb 5'11". I don't want them to be too loose, basically I'm trying to choose between medium and small. any help? maybe from someone who owns them? thanks!!