I sa a blog post on this the other day, give me 5 min to check browser history.
Ok, listen carefully. Mens sizes are made bigger. This means a mans small is probably a womans medium or something along those lines. Also, getting a bigger size doesn't guarantee it will be longer to cover your butt. I'd suggest a small. To be sure the best thing to do is get a measuring tape, ask someone to measure you. Then look at the chart at the bottom of the page and choose the appropriate size. This is the simplest thing to do! Cmon hop to it get measured!
I would buy a large, since it's a man size, it would cover, and great as a b/f sweatshirt over a tank and Capri leggings on a cool night, or over layering in the fall as a coat with a scarf.