I was just watching a documentary about England's navy and their battles and noticed that all the flags had the exact same proportions and sizes in the depictions of the battles. So I was wondering when or even if at some point flag sizes and proportions were standardized and if there are countries not following those standards.
The flag of the Vatican and Switzerland are actually square instead of rectangular. The flag of Nepal is two pennants.
Flags as we know them today are almost all based on European heraldry. The rectangular shape is a standard for all European flags. Rectangular flags are relatively easy to produce, recognise and are easily attached to a pole or even a branch. They followed relatively strickt rules which made it easy for sailors or soldiers to recognise each other. For example "colours" like red, blue, green or black are usually next to a "metal", depicted as yellow (representing gold) or white (representing silver). The proportions however differ widely, from square to 1:3.
The rectangular shape of European flags probably derived from Roman standards that showed their number.
Asian states used flags as well and had more room for different designs. After the British Empire forced the Chinese government to introduce a naval ensign they have chosen a triangular flag until they recognised that this shape was unique and changed it "to fit in".
hi! This question might be worth x-posting to the flag afficionados in /r/vexillology. Meanwhile, not discouraging discussion here, but fyi, here are some previous responses