Medium size is too small for me.
I'm male, I have a 34 inch waist, and I'm 66 inches height.  The medium belt doesn't even fully go around my waist. There is maybe like half-an inch of space still. Also the height of the device doesn't even prevent me from bending my back when sitting down.  I'll say it improves my posture maybe by 30-50% since it reminds me that I have it on when sitting down.  I would have preferred stiffer plates on the back, a longer length (height) of the device, and definitely about 2 inches more on the belt width.
Rating: 3 stars