I usually am between XS and S in For Days tops. I ordered the Small and it is still a tight fit. I am going to keep it as is but just be aware that if you want any looseness, you may have to go up 2 sizes.
Great quality fabric as usual with For Days
Rated -2 on a scale of minus 2 to 2, where minus 2 is Runs small, 0 is True to size and 2 is Runs big