Keeping It Cool: Done.
For reference: I'm 5'7-ish, 140 lbs. and I bought a medium.
I love the fit. It's not tight against my skin but it's not baggy or loose. Just right.
I cannot express how much I love the fabric. It's soft and super breathable. In the heat and humidity it keeps me cool better than any other shirt I've owned. I've also found I'm picky about fabrics and have returned several other shirts due to an itchy uncomfortable feel. This shirt does none of that. It's soft and comfortable down to the last thread.
I came back to backcountry.com to buy two more of these shirts.