Here is some corset advice I picked up some time ago:
“When you first start wearing the corset, do not lace it too tightly. Wait until the leather warms to your body temperature, and then start to wear it snuggly for short periods of time, a half hour to an hour to gently break it in. You should not lace it up tightly immediately as you need to give the piece time to shape to your body. Corsets should be fitted by waist size & bust size not bra size. Measure your waist at the narrowest point (the part that you would want to enhance and cinch in. Then measure your chest without a bra around at the widest point, usually across the nipple and then measure your ribcage just below the bust line. For all of these hold the tape measure snug but NOT tight and a hip measurement measured at the widest point and your hip measured at the widest point Then you should look for a corset that's closed waist is at least 2-6 inches smaller than your natural waist.”
I normally get corsets that are about 4-5 inches smaller (i. e. 22 inch corset for a 27 inch waist) but don't lace them all the way closed - so there is a 2-3 inch gap at the back. However I have been wearing my corset for years and it shouldn't be tightened more than about 2 inches the first few times. Wear it snug for short periods of time until the corset molds to your figure. However the MOST IMPORTANT thing I can tell you is to buy a good quality corset so that it will fit you better - poor quality corsets can do you damage. Its better to spend $200 on a better corset that will last you years
this is something you should email the retailer about. when it comes to corsets it's also important to get a quality steel boned one. if it's cheaply priced, then it's probably a piece of crap. I ordered one from Asia that was steel boned but the front closure was really cheap so it would open up by itself.