I went Medium. I have broad shoulders (44/54 in alot of French brands) and the jacket is just right in that dimension. This is due to it being almost entirely unconstructed.
Sleeves are a little short, but I don't mind the slightly-cropped look, particularly because the length is also cropped. No vents so the short length doesn't lean weird, on me at least.
That's if you're going for a tailored look.
And yea, it's polyester and there are some downsides there, namely a...