What size envelope should be used to mail a standard size business letter? ... Outside of the United States, the size is 5" by 11". You can use A4 paper for this. The number 8 is used for longer ...

Placing two paperback books in an A4 envelope results in a charge of £2.20 but when the books are put in separate small envelopes each one costs only 90p, a saving of 40p.