What is disclosed is a system for determining the size of a document on a platen of a document scanning device. In one embodiment, the scanning device comprises in part: a transparent pane (platen) whereon a hardcopy print is placed; a scan head comprising optical sensors which output a signal comprising an image of that hardcopy print; a plurality of LEDs positioned such that a topside of the platen is illuminated; and a processor for instructing the scan head to pre-scan the print. During the pre-scan, optical sensors comprising the scan head sense light from the LEDs of areas of the platen not covered by the hardcopy print at a first intensity and sense light from the LEDs of areas of the platen which are covered by the print at a second intensity. The processor determines the size of the hardcopy print from the first and second intensity levels.