I wear a large in most things, have a 42" bust, and I bought a 4X because of the size chart and customer reviews that I read before purchasing. I could have worn a 3X fine, but this still looked great. All of the corsets or bustiers that I have gotten from Amazon sellers before have been 3X or 2X. (mostly 3X) Just be sure you measure yourself and check the size chart for each listing.
This is much higher quality than the price suggests. It is very well made!