Dean,
I think their sizes are pretty dead on. I wear the XXL long in the stuff I buy. I have at least 8 of their jackets, some wool, some tincloth, and some both. I am a Filson junkie. That stuff is warm and wears like iron. On their coats when it says 48 or 50 chest-size, its pretty close. I even had a suit made for me and the chest sizes were the same as my ol Filson Double Mac Cruiser. Can't go wrong with Filson, and their field bags and duffles are indestructable. The only thing that is hard to size is the vest. Since I wear XXL or 50 long in all my other coats I ordered one of the mac vests in the same size...it was a tent, a very nice tent, but a tent none the less. I had to order a medium in the vest. I have big shoulders and my gut hasn't caught up, yet, so I didn't need a large there, yet.
-Hillbilly