Size of window relative to area of room

Does anyone know of a rule of thumb for sizing the area of windows relative to the area of the room? Also, would this rule vary depending on whether the room is north or south facing?
I'm building a house at the moment and want to double check that the windows are large enough.

the size of the openable section of the window is directly in proportion with the room in order to comply with ventilation regulations....
of the floor area of the room.

from what i remember theres also a light to floor area porportion of 10% to be achieved according to the building regs....