what is A4 size?
It's the standard used by most of the world. It is 210 mm × 297 mm = 8.27" × 11.69". The standard was adopted by the Soviet Union in 1934.
Paper size - Wikipedia, the free encyclopedia
In the US and Canada it isn't. We use "letter size", which is 8.5" x 11". In fact, the US and Canada are the only countries that haven't adopted that standard.It's the size of the paper in a regular school writing book.