This legging style by Southwest Legging Company features:
S/M fits 6-12 (aka One Size or OS) 27" waist (sold out, sorry!)L/XL fits 12-20 (aka Tall and Curvy or TC) 31" waist
This is an estimate, everyone's body is different. These have amazing stretch! Take your best guess. You can always return them (for free!) if they don't work out.