Classified as one of the best tools for marketing through mail, post cards are very appropriate and effective for getting your message to your customers fast. an excellent tool for communication. whether your choice is mail or personal. the post card is your best communication piece for the price. sizes vary from: 4 x 6 , 5 x 7 , 6 x 9 , 8.5 x 5.5 generally printed on card stock printed full color ink on one side and black ink on the postage side.