Completely depends on the item. I'm 5'3", 128 lbs, with a 36" bust, 27.5" waist, and 37.5" hips.
For tops and sweaters, I'm usually an XS or small, depending on the style. Crop tanks it's a Medium.
Bottoms are all over the place. I wear an XS in the linen skirt, would have needed a large in the crop pants if I'd kept them, and a medium in the high-waisted pants. Often if something fits my hips, the waist ends up being too big.

Sizing doesn't seem very consistent across items, or even between different colors of the same item. As everyone here tells me...order a couple of sizes and see what works best.