After reading the reviews, I decided to purchase one size up.  I am not a skinny girl - 5'8" 175 and I normally wear a size 12 - depending on the generosity of the manufacturer - so I ordered an XL in this Curvi Shapewear.  It fit perfectly without any discomfort and I wore it/danced in it for several hours.  I wore it without the straps and I had no trouble with the top sliding down - although because it is a skirt style the bottom will move up when you sit but it is easily tugged back down.  The shapeware tamed all my lumps and curves and made me look smooth under my dress.  I believe that I could have worn a size large but it may not have been as comfortable - but I think it would have been doable.  So depending on how snug a fit you dare to have...  My husband thought it was SEXY and it looked like a HOT dress (albeit skin tight) on its own.