A mail processing system that includes dimensional rating capability that provides true length support for carrier rates to prevent improper classification of mail pieces. The mail processing system determines a first dimension, parallel to the path of travel, and a second dimension, transverse to the path of travel, of a mail piece. The first dimension and second dimension are compared to each other to determine which is greater. The greater of the two dimensions is assigned to be the length of the mail piece, regardless of the orientation of the mail piece. The format of the mail piece is then determined based on a comparison of the assigned length and assigned width dimensions to maximum dimensions specified by a format table. Based on the determined format of the mail piece, a delivery fee is then calculated, and an indicium can be generated for the mail piece.