How do I choose the right bottoms size?

It depends on the piece. The Only The Best joggers come in S to XXL and list waist, hip, inseam, outseam and leg opening on the product page — start true to size at the waist and size up if you want a looser thigh or more length over the shoe. The Denim Skirt is cut differently and has no size options, so check its own product page for shape and fit.