I already own 3 of the one button wraps, and just bought 3 more. The sweater is very soft and comfortable. I would say this runs true to size. I purchased a XXS P, and they are perfect! I absolutely LOVE these sweaters!
I'm a size 4, 27, S and I got this in a S. I have thin arms and have room to wear a thin long sleeve shirt. I like it a little looser but if you want a more fitted look, I'd size down.
