Because everything is stretchy, most poema garments are extremely size-versatile, and will flatter a wide range of body types. This is also due to the ingenuity of the designs. The labels "extra small, small, medium, large" are general estimates. Taking your measurements with a measuring tape and comparing them to the listing's measurements is the most dependable way to know how a garment will fit you.
When the measurements on a listing say that a garment's waistband stretches to a certain number of inches, that number is general, too. For example, if you have 40" hips but you are looking at a skirt that only "stretches to" 38," you will almost certainly be able to get it on over your hips. Most garments actually stretch a bit more than the listings say (for the purposes of fitting over hips/shoulders).
For reference, Hannah is 5'9" and usually wears a small in tops and a medium in pants and skirts. She measures roughly 27" at the high/natural waist and 34" at the low waist (around the hipbones).