How do i measure my EYE?!?!(eye contacts question)?

ok, so, a friend and me are gonna buy a pack of colored contacts as a joke and have green eyes at school (well, one green and one natural blue). so i was about to buy them and it says "DIA" and under that "13.8". ??? does dia mean diameter? if so HOW ON EARTH DO I MEASURE MY EYE? i want these to work!!! so what say you on this one? (clearly neither of us wears contacts regularly:P)