I watched a program that talked about corsets. They claimed that the corsets worn in mainstream fashion back in the early 19th century are different as the ones worn now. The ones now are simply tight but are not intended in the long run to mold your ribs in order to have a smaller waist size like back then.
But on a size, they said that to start with, measure your waist, and try a corset an inch or two smaller. Although, you might want to check that on wiki, I was studying while I was watching the program on TV.