I am 5'6" and weigh 135, I ordered a Medium.

I'm a nurse and wear this shirt under my scrub top.  I LOVE THIS SHIRT!  It is super stretchy, so the ends of the sleeves don't get all stretched out when I have to push the sleeves up to my elbows.  It is perfectly tight and doesn't bunch up, and it's also long enough to cover my butt crack when I'm squatting down or bending over at work.  Dude.  For $6, I totally plan on ordering a ton of these.