Bioflow Sport products are silicone and designed to stretch over your hand. Please remember that the tighter the size you buy for your wrist, the more stretch it will require to fit over your hand, creating more pressure to the magnet housing seal.
Measure around your wrist using a tape measure. If you are between sizes we recommend that you go up a size. (e.g. if your wrist measures 18cm then you may want to choose a size Medium).