Good Sizes to remember in your designs

Dimensions
Minimum postcard dimensions 3 ½” x 5”
Maximum postcard dimensions 4 ¼” x 6”
Minimum letter dimensions 3 ½” x 5”
Maximum letter dimensions 6 1/8” x 11 ½” x ¼” thick
Minimum flat dimensions 6” x 6”
Maximum flat dimensions 12” x 15” x ¾” thick

First Class Mail Bills, business transactions and personal letters must be mailed first class
Weight
Up to 13 ounces (over 13 ounces mails Priority Mail)
Postcards
Minimum size 5 inches long by 3 ½ inches tall
Maximum size 6 inches long to 4 ¼ inches tall
Letters
Maximum size 11 ½ inches long by 6 1/8 inches tall by ¼ inches thick*
500 piece minimum to qualify for presort rates

* nonstandard surcharge may apply

Standard Class Mail Promotional mail and impersonal messages
Weight
Up to one pound
Postcards
There is no special category for postcards using standard class. In most cases it is cheaper and faster to send postcards first class.
Letters
Maximum size 11 ½ inches long by 6 1/8 inches tall by ¼ inches thick*

200 piece minimum to qualify * nonstandard surcharge may apply