I am 5' 6", 140 lbs, fairly trim build,...
I am 5' 6", 140 lbs, fairly trim build, live on the Canadian Prairies, so we get maybe 10 days of -35C with wind in January. What size would be best in this jacket? It should not be too loose because that is how you lose heat. I really like the profile of this jacket and the density.
You best bet will probably be to compare your measurements with the sizing chart below the price. They are usually pretty accurate.
you should go to Patagonia.com they have live chat with the people who know exatly how to size you in their garments.