VR is right - it's the viewers perspective. They are all actually 7' 5".
Seriously though, look here:
http://www.parliament.the-stationery.../63/63we06.htm
It's not really about height overall - no point having 5' legs, no body or neck and a 7" head for example. I understand that certain body parts, for entry into the Royal Navy, are expected to be less than 4 inches, whereas for entry into the RAF as a pilot an overall length of not less than 9 inches is required.
