First Class Mail Letter Size Envelope. But if you decide to create your own special envelope, or you design a piece to be folded to letter size, be sure to keep the letter dimensions in mind to avoid paying a higher price. All postcards and envelopes (or flats) must be rectangular, otherwise an additional charge may apply.
Max size in the stnd envelope series is #14 (5x11.5). Format and max measurements max weight 1st class 2nd class; We would recommend using an envelope no larger than c5 to fit these restrictions comfortably and avoid any excess costs.
Popular large envelopes used for mailing documents are 9 inches by 12 inches and 10 inches by 13 inches.
But if you decide to create your own special envelope, or you design a piece to be folded to letter size, be sure to keep the letter dimensions in mind to avoid paying a higher price. Ideal for mailing greeting cards, regular letters and postcards, the maximum size that would fall under a 'letter' is 165 mm x 240 mm. These require two first class postage stamps for the first ounce and additional postage for each ounce beyond one. We would recommend using an envelope no larger than c5 to fit these restrictions comfortably and avoid any excess costs.