I am a little surprised by this, but on the other hand, American Eagle shorties have some stretch in the fabric. I usually wear a size 10 in American Eagle pants and shorts (I have two pair of shorties, one khaki and one stretch denim.) I am 33" in the waist give or take, and around 37" in the hips.
I wear size 11 from Hollister, and in the (no longer made) Victoria's Secret shorts size 10 is a little loose on me.
I like my shorts with a little room underneath so I can wear them over a bikini bottom in order to use them for a cover up at the beach or pool. So you might want to factor in enough room for whatever you plan to wear underneath.