What is the maximum size allowed for EDDM postcards?

USPS allows EDDM pieces up to 12" x 15", at which point they are classified as flats rather than letters. Size is not the only limit, though, weight and thickness caps apply too.

Oversized formats command attention in the mailbox but cost more per piece in postage, so weigh the standout factor against the budget on high-volume routes before going large.