Paper A Series - Size (A1, A2, A3, A4 etc)
Paper size is normally measured to an international standard known as ISO 216: A3, A4, A5, B4, B5 and so on. These sizes all have the same aspect ratio: 1:√2. A4 is by far the most common and this is the default size for most office and home printers. A3 is twice the size of A4 and tends to be used more for posters or presentations.
The first form of communication between companies often comes in the form of a letter,
and therefore the envelope you use is vital in portraying your company’s image, along
with its stationery.
The most common sizes are:
C4 (equivalent to A4 paper size)
C5 (equivalent to A5 paper size)
C6 (equivalent to A6 paper size)
DL (fits a compliment slip)