Business firms use a large size envelope also known as a legal size envelope or a No. 10 size envelope. 

To fold a letter for a business size envelope,
first fold up the bottom third of the paper. (2)
Then fold the top third of the paper down over the bottom third.  (3)
Insert the letter with the last crease closest to the envelope flap.  (4)
When the letter is placed into the envelope in this way, the first thing that the addressee will see when opening the letter is the letterhead/heading.