At TimJeweler, we offer small to extra large sizes (US):
6.5 inches for small wrists (kids, teenager);
7.5 inches for regular wrists;
8.5 inches for large wrists;
10 inches for extra large wrists.
STEP 1: Measure your wrist with a flexible tape measurements or a strip of paper just below the wrist bone, where you would normally wear the bracelet.
STEP 2: If you are using a plain strip of paper, mark your size with a pen or pencil then use a ruler to measure the length, and give your wrist a loose fit to move around.