Fits true to size for a relaxed fit. For a less relaxed fit, if you're on the lower end of your size range, you can choose to size down one. For example, a size 6 could wear the small or XS. It depends how you like your shackets to fit.
Amy is pictured wearing a size XS. She is a size XS/2, bust: 34", waist: 24", hips: 34" and height: 5'5".