G'day,
The common paper size here is A4 which as Helen said is 210mm by 297mm. The actual sizes might not make much sense but the proportion is 1 by root 2. If you increase each side by a factor of root 2 the area doubles and that's the next size, or to put it another way one A3 sheet is the same size as two A4 sheets etc. Good system, even if the numbers work backwards. End of maths lesson.

Cheers,
Alan