A data processing device includes an input unit including an input detection region, a detection unit, a partition region setting unit, a valid region setting unit, a longitudinal direction determination unit and a magnification determination unit. The detection unit detects input to the input detection region. The partition region setting unit partitions the input detection region into partition regions. The valid region setting unit sets, among said partitioned partition regions, a partition region in which input is detected as a valid region. The longitudinal direction determination unit determines a longitudinal direction of the valid region. The magnification determination unit determines a magnification for enlarging or reducing a size of a drawing corresponding to the input in the valid region, so that a width along a direction perpendicular to the longitudinal direction of the valid region conforms to a width along a transverse direction of a printing medium.