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, wherein, the page background determining section determines that a plurality of page background types exists, the page background density value is compared with a predetermined white page background density threshold, and the document size determining section redetermines the document size when the page background density value is greater than the white page background threshold wherein the image processing apparatus further comprises a removal pixel-number determining section which determines a number of pixels to be removed from the candidates for page background pixels based on the redetermined document size, and wherein the page background determining section redetermines a page background using a number of pixels in which the number of removal pixels are removed from the number of white pixels.