So just what size can your post card be?

Postcard Sizes and Thicknesses

First Class and First Class Presort
Minimum Size: 3.5" x 5"
Maximum Size: 4.25" x 6"

Standard and Non-Profit Presort Mail
Minimum Size: 3.5" x 5"
Maximum Size: 6.125" x 11.5"

Thickness
Must range between .007" and .016" thickness

Aspect Ratio for Postcards

What is it and why it is important.
If the aspect ratio (length divided by height) is not within 1.3 to 2.5 (inclusive), the piece is a nonmachinable letter, which essentially means it will require more postage than a machinable letter.
For example:

• a. For a letter-size piece that is 4 inches by 6 inches, if the address is parallel to the 4-inch dimension, it is 4 inches long, which is less than the minimum length of 5 inches required, this piece is nonmailable.
• b. A piece that is 5 inches by 8 inches (and within letter-size thickness dimensions) is a letter. If the address is parallel to the 8-inch dimension, the piece is 8 inches long and 5 inches high. The aspect ratio of this piece is 1.6, so it is a mailable letter within machinable dimensions for length and height.
• c. For a letter-size piece that is 5 inches by 8 inches, if the address is parallel to the 5-inch (shorter) dimension, the piece is 5 inches long and 8 inches high. The aspect ratio of this piece is 0.625, which is not within 1.3 to 2.5, so it is mailable as a nonmachinable letter.

Nonmailable and Nonmachinable Placement of Address

The placement of the address on a letter-size mailpiece may render a piece nonmailable or nonmachinable. If the length (the dimension parallel to the address) of a letter-size mailpiece is not at least 5 inches, it is nonmailable. If the height (the dimension perpendicular to the length) of a letter-size mailpiece is not at least 3-1/2 inches, it is nonmailable.