Egress windows (bedrooms)
For the most part, the National Building Code isn't that concerned about the size of windows - except for bedrooms.
The National Building Code mandates that bedroom windows have at least .35 square metres of area, with no dimension smaller than 38 cm, when the window is open. (For you imperial folks, that’s 542 square inches, and 15” minimum opening).
The "when the window is open" part of the above is critical. Here's why.
Here you’ll see two pictures of a window: the 48"-high window occupies 16” when closed, but because it’s a casement window, narrows to a bit more than 12” when opened: this window would not be code-compliant as a new installation in a bedroom.
The idea is that in the event of a fire, someone in a bedroom may be trapped by flames or smoke: a window of suitable dimension will give bedroom occupants a chance to bail out the window. More accurately, the window sizes are based on an average firefighter in full SCBA gear: the windows are as much for firefighter entry as they are for occupant escape - which explains why our inspectors will check for egress dimensions on second-storey windows.
For the most part, all a builder or contractor need to do is specify an egress-compliant window from the supplier - but it's wise to check before installation.