The size of a postcard is 4-1/4 inches high by 6 inches wide, maximum, to be eligible for First Class Mail postcard rate by the USPS.
For the USPS, minimum postcard size is: 3-1/2″H x 5″W.
Also, the USPS requires that postcards be a thickness of 0.007 inches, minimum; and 0.016 inches maximum.
Lastly, the mailing piece needs to be rectangular.
It makes sense to design your postcard to meet the USPS standards. The First Class Mail postcard is given a very low rate by the USPS, and receives all the benefits of First Class mail, including mail forwarding and return if addressee not found.
A footnote — please note that our Postcard Magnets at magnetbyMail.com are not mailed using US First Class postcard postage. Instead we mail most using rates based on bulk mail. For more info about direct mail campaigns with magnet mailers, check us out at magnetbyMail.com . Or drop us a postcard!