These run a bit big (if you're relatively slim). I got a size medium, and the actual chest (pit tp pit) measures 41", which is on the larger side of medium IMO, though maybe it will shrink a bit after a couple washes. I bought them mainly because the arms are not huge -- the arm-hole opening is 6.5", which means the arms don't stick out in a big triangle and flap around. Otherwise it's a pretty basic, lightweight t-shirt that's decent value for the money.