Darksider wrote a review of Stoic Merino 150 Crew - Long-Sleeve - Men's on November 11, 2009
I am 6'6" with a 48" chest and I went with an XXL--I am glad that I did. It has a nice athletic fit, without being skin tight which is what I wanted. The body is a great length, and the arms are even long on me (which I like)!
If you are familiar with the Backcountry.com long-sleeve merino crew shirts, you will be happy to know that the thumb-holes are usable, and in the correct spot.
Finally, the weight. I would estimate the weight to be about 120g/m2 (maybe 140g/m2 at the most). It feels about the same weight as the Backcountry.com short-sleeve merino shirts or a SmartWool microweight. The merino soft, and of good quality.