I feel like sites that have 1x, 2x, etc. sizings are less "serious" corsetry items, and if you're looking for a legit, well-boned, actual corset that someone can lace you up in and give you a scary/hot wasp waist that takes your breath away - you'll have to go to a place that makes them custom or at least has the sizes in inches. Those places typically have a very active webmaster/owner whom you can contact and make sure of what you should do, since they're usually awesome small businesses. I've seen a bunch of ads of affordable handmade corsets on Fetlife but the names escape me, if I can remember I'll post.

I remember biggalslingerie.com having 1x etc and there the 6 is not deducted, since it's not super hardcore corsets, just some sexy take-some-inches-off-ers. The only advice I have is, watch for the length because if you have a large stomach you might not want anything hanging out at the bottom, but also you won't be able to sit if it's too long, from it digging into your hip bones/thighs. That and, if you're not sure, find a site that has a person who will talk to you at least a little about each one (for example, if the maker is willing, send your measurements, maybe a body pic, and the styles you're considering and see if they have opinions about why which one will/won't work for your body). If that's too much work, at least make sure you send your measurements and make SURE it's the right size!

Hope that helps, I know it's a bit general and simplistic