I have several of the mia shirts, even though I really only need 1 or 2 since they are wool. The mia is lightweight and perfect for the summer. I find the fit slightly looser than Addie due to the cut. I have tried both small and medium, and ultimately chose medium for a looser fit. Since I find it slightly too big, I actually put my medium Mia in the dryer, but hang dry Addie. For layering, I'd go with a small. I have a midweight longsleeve that I primarily intended to use for winter sports and a small is perfect. The heavier weight also holds me in a little better, so the smaller fit works. For reference, 5'5", 130#, 32DDD.