To ensure the correct fit, first measure your wrist with a tape measure. Wrap it leaving the tape measure flat and tight around your wrist. Then, add 1/2 inch to the measurement. This should allow for a snug fit, but also allow for a small gap giving your wrist some "breathing room".
Aromatherapy Bracelets sizes are the length of the bracelet in inches. If you measure your wrist size with a tape measure and then add 1/2" to 1" to the length (based on how close you would like the bracelet to fit), you should be able to choose the perfect sized bracelet.
If you do purchase a size that doesn't fit well, Beads-N-Style is happy to provide a Free Exchange for the correct size.