Roughly put, a letter-size direct mail is any rectangular mailer at least 5” long and 3 1/2″ high. However, a mailer larger than the letter-size maximum dimension of 6 1/8″ x 10 1/2” is considered as the “fat” size direct mailer. The “flat” size mailer cannot exceed 12” x 15”. The postage rate for a “fat” size mailer is significantly higher than the postage rate for the letter-size mailer.