A:
JawboneUPSizingChart (from amazon.com)
Step 1: Get a string, a marker and a ruler. Wrap the string snugly around your wrist. Mark the string where the two ends cross.
Step 2: Use a ruler to measure the string from one end to the marked place. Then see the size guide to the right to determine which wristband is right for you. If you're between two sizes, we recommend choosing the larger size.
Small is 5.5-6 inches
Medium is 6-7 inches
Large is 7-8 inches