They run big and long.  I normally wear size medium or even large because I like my tees to be on the loose side, but these in small are quite roomy, I could size down to XS (I'm 5'5" and 130 lbs).  I bought one of these over a year ago in Bloomies in size small which is shorter in the back than these on Amazon, which I like better.  Overall this is a quality tee, flattering shape but too expensive for a basic.