There are many sizes of paper, but the most common system nowadays is the A measure, whereby each size is half the area of the next lower number (you fold in half along the long edge to get the next size down). In this system, the commonest size for office stationery is A4, which is 297 mm by 210 mm. The base measure of A0 is 1188 mm by 840 mm, which is very nearly one square metre.