I'm 6'3 180, picked up an XL. It'll be the only XL in my wardrobe, but frankly I'm just hoping I can fit into this piece.
You should be fine. I am a medium (actually, I am the medium), and I am 5'11" and just shy of 165 lbs.
I'm thinking about picking this up in an XL to wear as a shirt jacket. How will the fabric breathe in the summer?
Like heavy jeans. The weight of the shirt makes it best worn as a shirt in winter and as a jacket in the spring and early fall.