USPS - Maximums and Minimums

Letter Size: 6 1/8 x 10 1/2
                    3 1/2 x 5

Post Card Size:  4 1/4 x 6
                          3 1/2 x 5

Font Size: 18 pt maximum, 8 pt minimum
                 (All caps preferred)

Font Type: Sans Serif, no kearning
                  (Helvetica, Geneva)

Font Style: Plain