I'm 5'6 ~125 lbs and ordered this in a Small. It fits as expected (sort of a medium-loose casual feel) and is really comfortable.
The material isn't flannel or tshirt material, but is more of almost a 3star hotel bedsheet feel if that makes sense. I was a little offput by it at first but it's so cute and comfortable that I bought it in another color as well.
I wear it tied and with high waisted jeans.