I'm 5'4" 197lbs and hourglass shape. I wear size 14 pants and bought the Large. Fit is good and comfortable. I needed this for a short bridesmaid dress. I like that I can pull the slip up/down on my abdomen to adjust the lenght.
This is an okay item but definitely not great. When I first bought it I loved it but over time it loses its form. So it works for a month or two and then you might as well throw it out and buy another one. On the plus side, the material is of very good quality. You don't have to worry about it ripping easily. I own several and none of them have ever ripped.