postcard specs for USPS

Physical Standards for Cards Claimed at Card Rates
1.2.1 Dimensional Standards for Cards
Each card (each stamped card or postcard or each half of a double stamped card or postcard) claimed at a card rate must be:

a.
Not less than 3-1/2 inches high, 5 inches long, and 0.007 inch thick.
b.
Not more than 4-1/4 inches high, or more than 6 inches long, or greater than 0.016 inch thick.
c.
Rectangular, with four square corners and parallel opposite sides. Card-size pieces meeting the standards in 1.2.1 may have finished corners that do not exceed a radius of 0.125 inch (1/8 inch). See Exhibit 1.1.1c.

http://pe.usps.com/text/dmm300/201.htm#wp1042477