When designing a postcard for printing, there are a few guidelines that you have to adhere to. The smallest size your postcard can be is 3.5” by 5” (the size of an index card). Any size smaller than this will not be able to run through the US Post Office. The largest size a postcard can be (and still be considered for the postcard pricing of 24 cents an item) is 4.25” by 6”. A postcard must also be a minimum thickness of .007 inches (this means that you cannot send a piece of paper cut to size as a postcard). You will also need to design your postcard with both a front and a back, and the address must be readable on the back side. You'll also want to make sure to design your postcard in a program that your printer accepts (for example, any of the Microsoft Office suite, Quark Xpress, or Adobe file formats). Check with your printer to make sure that your file format is accepted before sending any digital files.