To determine measure from nose to base of caudal fin. Kings and Giants are determined by body length-only.
According to the International Betta Congress (which sets guidelines by which Betta are judged, normal Betta should be a minimum of 1.5" body-only to show and Giants 2.5". As far as I can determine, they do not recognize "King" Betta. That is pretty much a marketing name invented by PetSmart or PetCo for Betta which fall between the two recognized sizes.