You will need to measure yourself first before picking your corset size. Here, we briefly decribe how and where you should measure for different types of corsets;
Length - sit on a hard chair and measure straight down from right below your bust to the fold where your lap starts.
Under Bust - measure right below your bust (where your bra band sits).
Waist - measure the smallest point of your torso (usually just below the rib cage or about 1 inch above your belly button). Note that you should stand and breathe normally when taking this measurement. When choosing your corset size, this is the primary measurement you should look in to.
Hip - measure around your hip bone, not around your bum.