Deciding on the most accurate size to order can be difficult.  If you are able to measure the wrist of the person this is intended for, you will want to use a flexible measuring tape.  Measure the wrist, just above the wrist bone (where a watch would be worn).  Once you determine the wrist size, you will want to add about an inch to that measurement, for an adult and about a 1/2 inch, for infants, toddlers and older children.  Measuring a bracelet that you already own and like the fit of, is another decent way to determine sizing (just be sure to measure end to end, including the clasp).  Keep in mind your preferences, as well.  If you like your bracelets to fit loosely, add more to your measurement.  If you like them to fit snug, add less.  A good "standard" bracelet size, for adult women is approximately 7 1/2 inches long.