First of all, let’s address the fact that ‘standard window sizes’ are actually a myth. There is no industry-wide standard sizing, especially when so many windows are custom-made for houses. Standardisation of window sizes differ between manufacturers, who will make windows in sizes that they deem to be standard.  As windows and houses are now mass-produced, this has led to an industry-wide standard of sorts, but again, it’s at the discretion of the manufacturers to decide what they think is standard. Those “standards” will also change over time.