I'm 6' 1" and 176 lbs. I bought the large and that size fits me perfectly. Not too tight, and not too loose. But to be clear, this is not a skin tight base layer type of shirt. The material is very soft to the touch, but certainly not flimsy.
The feel is fantastic. This shirt would be perfect for either a base layer in freezing cold temps, or as a stand alone shirt for temps in the 50's. The material wicks perspiration well, and I love sporting the Sporthill brand. As of now I own both the blue and the gray colored shirts. If it came in more colors I would likely buy more of them, and I suppose that is the best testimony, right?