Love this coat! I was unsure on size between a small and medium so ordered both to be sure. The fit on the arms is about the same for both the only difference really being the width. I went with the size small - I am 5’5, and about 115lbs. It’s very warm and cozy. I can comfortably wear a crew neck sweatshirt underneath (though you really don’t need to as it is so warm). Would recommend to anyone!
It's a nice and warm coat, but as someone else stated, the arm holes are small and tight when putting the coat on, and I'm small-boned. I normally wear a medium but a large would probably feel better. The medium works as long as I remove thick layered clothing first, but really wish I had upsized.