Traditionally, shin guards are measured from the center of the knee cap to where you want the guard to end. Unfortunately, not every company uses this particular metric. Some use from the center of the knee cap to the very bottom of the shin guard, some are from the knee to the bottom edge of the hard plastic, and for good measure, some manufacturers may round their figures. (Sorry, I know that probably doesn't give the exact answer you were looking for.)
With all that said, as a general rule, most referees seem to fall into the 15" shinguard size. If you are on the shorter side, then go with the 14", and if you are on the tiller side, the 16" would probably be better.