A letter envelope is typically an A10 or size 10 envelope. This envelope size is one of the most common sizes that you will encounter. They will fit a standard 8 1/2" x 11" sheet of paper that has been tri-folded. These envelopes are used for personal letters and mailings as well as many types of business mailings. In addition to letter envelopes, legal-size envelopes are often used for business mailings.