@Pure Cane Soda:
from http://en.wikipedia.org/wiki/Paper_size :
"This standard has been adopted by all countries in the world except the United States and Canada."
so I would call A5 and A6 a world wide (well, mostly) known precise measurement.
Talking about what you think is an precise measurement: http://en.wikipedia.org/wiki/Inch
"An inch (...) is the name of a unit of length in a number of different systems(...). Its size can vary from system to system."
from http://en.wikipedia.org/wiki/Paper_size :
"This standard has been adopted by all countries in the world except the United States and Canada."
so I would call A5 and A6 a world wide (well, mostly) known precise measurement.
Talking about what you think is an precise measurement: http://en.wikipedia.org/wiki/Inch
"An inch (...) is the name of a unit of length in a number of different systems(...). Its size can vary from system to system."