To facilitate printing of postage or other images onto media of variable size, an apparatus determines the size of the medium in at least one dimension prior to or during the printing of the image, such that the image to be printed is adjusted based upon the determined size of the medium. The apparatus may be a postage meter, in which case information is received by a data processor concerning the size of a mail piece and the postal value. An initial image made up of a fixed portion, including postal indicia, and a user-defined variable portion, is then altered in view of this input to provide a final image for printing on the mail piece which fits fully on the mail piece. The alteration effects only the variable portion of the image, which may be truncated, scaled or replaced with a different variable portion.