Handy Paper Size guide. The UK has the standard universal "A" measurement of paper. An A1 sheet of paper when folded in half is an A2, an A2 sheet of paper folded, is A3, and so on.
Your standard photo copying paper is A4, the folded size of A4 is A5.
Interesting fact : If you weigh 16 sheets of A4 paper you can calculate its weight in gsm.