Like many others, once putting this on, you won't want to take it off. I purchased this for next summer, but have been wearing it to sleep in nightly. Just ordered another. I have a lot of Smartwool items, but WoolX has quickly become my new favorite.
For sizing - I am 5 ft 7, 145 pounds, broad shoulders and wear a Small in the other Woolx tops (Hannah and Jenny), but a Medium is a better fit in the tank as it is more form fitting. WoolX has the right about of stretch and softness.