I love my Cool Wings.

TE provides sizing information based on your upper arm measurement.

http://www.teamestrogen.com/prodDE_CW.html

Last year when I bought a new one, I ordered a medium based on my arm measurement but it was a bit small across the back. I was worried that the large would be too loose in the arms but I ordered one anyway and it ended up fitting fine.

If you're not sure what size to order, call TE and they will advise you.