It's best to select the size you most COMFORTABLY wear and buy most often. The size charts are just a guide to show how hips and waist vary in size for each shape.
Measurements reflect skirt lengths and approximately where they fall on your leg. Lengths are not exact, this is just a guide. Lengths will vary according to where you like your skirt to sit on your waist. Use your favorite skirt in your closet as a reference.