The size of the flap on a #6 3/4 Remittance is 3 3/8" x 6 1/2", or 3.375" x 6.5". The back flap is 1/4" shorter than the front of the envelope. Attached are templates of both the front and the back of a 6 3/4 Remittance. Here is the link for envelope templates https://www.4over4.com/blank-templates/envelopes