A proper corset should NOT be sized, small, medium and large. That's a poorly designed product and not really a corset at all. To get a corset to fit properly the sizing needs to be more specific.
If you are trying to buy online, e-mail the seller and ask for more details and their advice. No one can reliably tell you what corset to buy with no more information that your waist measurement and 'small, medium or large'.
Maybe, if you need to ask this question yet again, you could include a link the product you are considering? That might help. [ Razhie's advice column | Ask Razhie A Question ]
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.