If you're ordering these online some of them have a sizing chart. But I know brands can vary a lot. I've heard they mostly come in numbered waist sizes but mine actually came in S,M,L, XL kind of sizing. A tape measurer will be a big help. Maybe shop for a pair at a local tack shop and then once you find a brand you like you'll know what size works best. Especially since you're getting back into the feel of breeches. I was 10 the last time I wore breeches (wore in jeans since then it felt simpler) but I recently gave in to buying some a week ago just in case I go to shows.