1. An image processing apparatus provided with a conveying unit that conveys an original document, and a reading unit that reads the original document that is conveyed, the image processing apparatus comprising: a detection unit that detects a leading edge and a trailing edge of the original document that is conveyed; a specification unit that specifies a size of the original document, based on timings at which the detection unit detected the leading edge and the trailing edge; a determination unit that determines whether image data read by the reading unit is a color image or a monochrome image, for each size of a plurality of standard-size printing medium; and a selection unit that selects, from among a plurality of determination results obtained by the determination unit, a determination result obtained by performing determination using a size of the standard-size printing medium similar to the size of the original document specified by the specification unit.