In Australia, the European continent, the UK, they all use A4 paper.
In the US we use letter.
( pretty picture of paper size comparison at Wikipedia )
A4 is a result of a rational design process ( just like Helvetica, you know the font that rules your life that so much that you don’t think about it ). Letter was a colloquial guess at how long and wide a sheet of beat tree-pulp should be.
I, personally, like A4 a lot better. Like most continental europeans it’s slender and allows room for perspective. Letter is short and squatty, by comparison ( analogy applies as well ).
Notably, our favorite faux-Brits, the Canadians, are on the US system. I suppose this is a symptom of economic imperialism.
Yet if that were so, why is Mexico on the A4 standard? Maybe Canadians aren’t as not-American as they like to proclaim ( loudly, and often )?
Better run, I hear the sound of ice skates, it might be an skate-by assassination.