Ok so going by reviews,I purchased the Xs.  Normally, Im a small to medium in blouse, however, when I buy any workout shape-wear products,  I go small so that, needless to say, the shaper tightens enough to shape. A lady here posted that shes a medium and bought small and it fits great and she suggests you buy a size or two smaller so it can be tight.  So going by that and also that shes slightly bigger than I am, I purchased the xs.  Okay so heres the pro and con going two sizes smaller....
Pros: a much smaller size will certainly tighten your waist, help trim waist likely sooner especially if you workout, decreases appetite, and you sweat like crazy.
Cons:  I tolerated this well and left it on about nine hours but for most, it can be painful to wear.  You cant breathe well, and you're likely to remove sooner and can even pass out depending on how tight.  Lastly, putting it on took some effort.
When I first opened it I laughed at the tiny size and it did not go past my sides....seriously!  I later placed under my feet and pulled on it from the other end and it stretched that quickly.  I still struggled a bit to put it in but not too much.
So for me the size was too small but I made it work and I love it!  But I don't think people should go down two sizes.  You should go down gradually and more comfortably.  I have another I use and will continue to use this one occasdiknally as I lose the waist fat but its not necessary to tighten your waist this much.  In fact, if u plan on using it to go out, just get your size and no less because tightening too much makes a little back fat pop out and shows under clothing.
So I love it to workout in....speed walking, light running, etc...nothing too strenuous with this.
But overall, I absolutely love the string yet soft and durable fabric.  Definitely recommended!