I recommend wrapping a tailors tape measure around your wrist to measure. If you don't have one use a string/ribbon--wrap it around your wrist and lay the desired length across a ruler!

When selecting your size, keep in mind where you'd like your bracelet to fit on your wrist. For example the higher up the arm, the larger bracelet you may want. This is also good to think about when planning to stack with a watch or multiple bracelets!