Well, I'm usually about 31 inches in the waist, but I always wore medium men's unders because I don't like tight elastic on may waist. Medium women's panties can be a bit too roomy in the rump, though. Still, small sizes are usually too small. Pretty much I have to shop around, since fit varies a lot between brands and styles. The best fits I've had have been with control panties, such as those from Victoria's Secrets.
~ Lyric ~