The maximum size for a USPS flat size mail piece is 15x12x0.75 inches. Most of the time the issue is that disqualifies a mailing from being classified as a flat is the thickness but the length and width can also be an issue. Surprisingly there are no standard envelopes that are at the 15x12 maximum size, the closest is a 15.5x12. There are certain things that can be done if an 15x12 envelope is needed but the options would depend on the mail piece and desired look.

If one of the dimensions is larger than 15x12x.075 the piece no longer qualifies as a flat but instead would need to be sent as a USPS package which can increase the postage costs significantly. It is best to design a mail piece that falls within the USPS requirements for a flat which include the maximum size of 15x12x.075 but there are also additional requirements which help ensure the mail pieces can run through USPS automation equipment. It is best to mock up a sample and have it reviewed by a mail house or the USPS to ensure the specs are within the USPS guidelines