The size of the flap on a #9 Remittance is 3 5/8" x 8 7/8", or 3.625" x 8.875". 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 9 Remittance. Here is the link for blank envelope templates https://www.4over4.com/blank-templates/envelopes