Paper sizes
From Craft Wiki
Paper exists in a number of different standard sizes.
ISO A4 to A6
Most of the world uses the ISO standard A4 size paper. This measures 210 mm x 297 mm.
If you fold the paper in four, you would have a standard A6 card size, which measures 105 mm x 148,5 mm.
US letter to A2
The United States and Canada are the only two countries that officially don't use the ISO standard, but use the US letter paper. This measures 8,5 inch x 11 inch.
If you fold the paper in four, you would have a standard A2 card size, which measures 4,25 inch x 5,5 inch.