Envelope Size Chart. All regular envelopes feature a solid front with no window, and a basic flap on the back. They usually have a commercial flap and are most frequently used for letterheads, invoices, statements, checks, writing stationery and direct marketing mailings.

The c4 envelope size can easily fit unfolded a4 paper; All regular envelopes feature a solid front with no window, and a basic flap on the back. Dl (110 x 220mm) envelopes;