Minimum Size Standard
The smallest dimensions permitted for all mailable matter or for a specific mail processing category or specific rate. Except for keys and identification devices, a mailpiece less than ¼ inch thick must be rectangular and meet the following minimum criteria (otherwise it is non-mailable and must be returned to sender):
At least 5 inches long.
At least 3-1/2 inches high.
At least .007 inch thick.