1. An image processing apparatus comprising: a page background determining section to which document image data corresponding to a document image of a document having been read is inputted and which extracts candidates for page background pixels from the input document image data and determines a number of page background types; a white pixel extracting section which extracts white pixels from the input document image data; and a document size determining section which determines a document size of the input document image data, based on the number of page backgrounds types determined by the page background determining section, a page background density value which is the lowest density value in one or a plurality of page background areas determined to be page backgrounds, and a number of white pixels extracted by the white pixel extracting section.