It's the standard here in the USA. The other standard is the US Legal at 8.5 inches by 14 inches (216 mm by 356 mm). This is what is used in court settings (hence the name) but also things like paper mortgage statements will typically come printed on that. That is much similar to your A4 size.

I am familiar with A4, A5 and such. But I think that fewer and fewer people are. It's just not something used every day.

As a side note, most of the big important house bills and statements I still insist on receiving via US mail for protection reasons. There is a risk if I only had them emailed to me that my wife would not have access. If I were to suddenly die, I don't want my wife with our kids to miss a critical bill. By having them show up at the house in physical form provides a bit of defense in depth here.