I got this in a Small, but would be just fine in XS as well (regarding loose fitting). The length of the back of this hoodie is super cute and cropped to the perfect level. I'm 5'1", 120lbs. Very very comfy and love the hood.
Rated 1 on a scale of minus 2 to 2, where minus 2 is Runs Small, 0 is Fits Great and 2 is Runs Large