they make them in waist sizes, so you'd have to decide what waist size you want. I've got a 26 inch, normally, but i usually crank down to a 20. One thing you want to remember, when looking, is you don't want to have the corset closed in back, you want about an inch open at the waist.
http://www.nightshadecorsets.com/
this is a good site, I have a couple of corsets by her, and she just rocks. If you have questions regarding the notsowellendowed, she can suggest corset styles that will work for you, just have to email her. As one of the notsowellendowed, I'd suggest a victorian (comes up higher than an edwardian) or just a waist cincher (allows for the lovely vicki to work her stuff). Oh yeah, please keep in mind, her prices, if she doesn't have them in american dollars, are in canadian dollars.