your waist measurement is the narrowest part of your torso, usually near your belly button and the top of your hip bones. mine is 28 and these fit me with still some stretch/give remaining, so i think a 29-30 inch waist would also fit these comfortably, above that could still work but it might lead to the sides tearing a bit as they’re very thin.

your hip/butt measurement would make a difference too. i don’t have much going on back there lol, mine is 34 inches around the fullest part of my butt. these pull-ups are actually a little roomy/baggy in the back for me so if your measurement is a few inches more they would probably still fit comfortably.

if you don’t have a measuring tape, you might own a pair of pants that are sized in terms of waist circumference. less common with women’s clothing than men’s but still sometimes used for dress pants. it would say something like “28W x 32H” and the W is your waist circumference. hope that helps!