Fobskeys are offered in a 9" and 11" loop option.  The 9" option fits most.

To find the size that is best for you, measure out a 10" and a 12" piece of string and place the ends together. Pinch down about 1/2", creating a 9" or 11" loop. Fit your hand through to see which fit you prefer. Use can also use a soft measuring tape.