Here is an easy trick to check U.S. letter mail aspect ratio. Just divide the width of your envelope or postcard by the height. If your results are great than 1.3 and less than 2.5 you are good.
Why is letter mail aspect ratio important? It’s the key to getting the lowest postage rate, by ensuring the package can run through the USPS automated mailing equipment. When considering the aspect ratio you must also follow the sizing guidelines from the USPS.
Please use this as a guide only. To make sure that your documents follow the proper sizing guidelines check with your local post office.