I wear size 6 dress. I ordered a small and it fit perfectly.  I know its suppsed to be tight, but the first two days were a struggle. After a month I lost 4 inches from my waist with little exercise. I WOULD NOT SLEEP OR EXERCISE IN THIS. I would probably pass out.  In order to maintain your new shape you have to make a diet or fitness change unless you want to wear it the rest of your life.  I hadnt worn it for about 2 weeks (and ate more junk food) and my belly started spreading back to pre-waist trainer shape.