You would measure for your corset like you would measure the bandwidth for your bra.

Standing in front of a mirror, wrap your tape measure snugly around your chest, just under or at the crease beneath your breasts and flush against your back. Check in the mirror to make sure the tape measure is parallel to the floor and flat across your back. You may need a friend or partner to help. Record this measurement in inches and write it down. Now you'll need to do some basic maths to find your proper band size.

If the number is even, add 4 to get your Band Size
If the number is odd, add 5 to get your Band Size

Example: If you wrote down 32, add 4 and your band size will be 36, this will also be your corset size.
If you have a DD cup size or above you would go one size larger.