This is another exciting topic, lol. Head high is normally 6 foot and double o/h starts at 12 foot. Five foot is shoulder high using a standard 6 foot person, unless we want to weenie down to 5 foot standard by using the 3rd world mean height of men or round up/down to 5 foot for women. Think about it for a second... surf in the 8 to 9 foot range could be called almost double overhead. NOT. Just say how many feet high the face of the wave is and forget about comparing body part heights that are so inconsistent when there a range of menehunes to The Beef (that MD freak of nature for whom a 9' board is a shortboard) out there riding.
wave heights have always bothered me....why is it that a 6 foot wave can look like a 4 foot wave from a distance, a 9 foot wave up close, a 3 foot wave to a Hawaiian, or head high to most of us east coasters??? The difference between face heights, back heights, swell heights (crest to trough) has always bothered me too. A 4 foot swell can have a 10 foot face if conditions are right !!