I'm 6'3" and 240 lbs and purchased the XL which fit true to size. The sleeves are just a tad to short but almost every long sleeve shirt I try has the same promblem - we call them monkey arms. The shirt is a midweight and good for 20 degree and up weather as an undershirt. I like the feel of the material and it washes well. I would not call it a "wicking" shirt as the material is similiar to cotton.

EDIT 1/23/13;
I wanted to update my review and correct what I said about it not being a wicking shirt. I recently started having night sweats for whatever reason and literally waking up in a puddle of sweat that soaked my sheets (I know, yuck). I started wearing this shirt to bed and it 100% solved the problem. It DOES wick away sweat and performs the way it should. I just bought 2 more today and will get the black just as soon as it comes back into stock in my size.