A Quick Ref to Paper Sizes
The easiest way to approach paper sizes is to start with a plain piece of computer printer paper. That’s your A4 of course – the standard business-letter size.
The A series paper sizes is the internationally used standard paper size. And it's pretty easy once you know this little trick: successive paper sizes in the series (A1, A2, A3, and so forth) are defined by halving the preceding paper size.
So an A4 is half the size of an A3, which is half the size of an A2.
There are also standard envelopes to go with standard paper sizes – if ever you've seen ‘DLE’ used, that’s because it's the standard envelope size (110 × 220mm). It’ll fit a sheet of A4 paper folded into thirds.