I am 5'9" and wear a 40R jacket. This coat in Medium fits me perfectly. It will be fine wearing a sweater underneath, but probably not a sport coat since it isn't that roomy. The sleeve length was perfect for me. But if you are 5'11" or taller, the sleeve length of the Medium will be too short for you. The detachable inner rib looks like a scarf and snaps up the front; it will keep the cold air from entering through the buttons on the front of the jacket.
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