None of the companies who make these back supports seem to have ever tried their products on real people to get the sizing right. I don't see any size guidelines in the listing and the size chart offered in another review is pretty far off, from my experience. I have a waist size that varies from 42 to 46 depending on how active I am. Because the stretchy side wings are too long on the XL, it's really too big to give me strong back support. I just ordered the same thing in a size Large, which I expect to be pretty tight when my waist is at 44 inches -- but it needs to be tight to provide good back support. This brand is very well made but it's useless if it doesn't fit right. I could probably have used the XL if they had simply made the side panel adjusters a bit shorter. They were definitely too long to give any meaningful adjustability -- so you have to be within a few inches of the size it's made for or you will need a different size! As I said, this back support is very well-made but, size-wise, this company has not planned or designed very thoughtfully.