Go to LBS and try stuff on. Even if you don't purchase, it will give you an idea about how they fit. Me? I'm exactly between a med and lg for most jerseys. Even though I can fit into it, med tends to be a little too uncomfortable, so I bump it up one size. Even shirts of the same brand fit differently. I have 2 L-G shirts and one fits well and the other is slightly big. IMO it's always better to try it on before you buy.