Sizing a corset is simple. The size numbers refer to bust size. If you are a bust size 40C or D, you would wear a size 40 corset. If your cup size is larger, you should go up one size. In this case, up to a size 42. Sizes do range a bit within corsets so if you end up ordering a plus size corset, or regular corset online, make sure you purchase according to measurements, not according to “typical” sizing.