I am petite at 5'2" and 110 lbs, and normally wear an XS but in this coat I got a Small. I work at a dog daycare in the Seattle area and needed a coat to keep me warm and dry, yet also have the ability to layer if I need to (in the winter I definitely do). The coat keeps me extremely warm, I just wish the hood had a fleece lining or something of the sort; it's the same material as the entire rest of the coat. I do love that the inside of the pocket have the soft inside, though.
If it wasn't for my job and the fact that I am outside for most of the day, I definitely would have ordered an XS, otherwise the Small is perfect if you need it for long periods of time and want to layer.
Overall I am extremely happy with this coat! It is excellent quality, and I like that it's actually semi-fitted and shaped to fit my body. I am expecting it to last me quite a long time (although with dogs frequently jumping up to greet me it may not last as long as someone who uses it for hiking or other outdoor activities), and when it does decide to fall apart, I will definitely buy another!