1. An image forming apparatus comprising: a document reading section configured to read one side of each of mixed original documents of different document sizes constituting a plurality of pages to generate respective image data sets of the original documents; a document size detecting section configured to detect respective sizes of the original documents to be read by the document reading section; an image forming section configured to form images on both sides of recording media from the image data sets of the original documents generated by the document reading section; a recording medium size selecting section that when, out of the image data sets of the original documents generated by the document reading section and constituting the plurality of pages, one of a pair of image data sets to be placed on one and the other sides of a common recording medium by image formation of the image forming section is greater in length of the document size detected by the document size detecting section than the other image data set, the length being a dimension in a direction of conveyance of the recording medium, selects, as a size of the recording medium for use in forming images from the pair of image data sets, a size having a greater length in the direction of conveyance of the recording medium than the other image data set; a conveyance section configured to convey the recording medium of the size selected by the recording medium size selecting section to the image forming section; and a control section configured to allow the image forming section to form, on the recording media conveyed by the conveyance section, images from the image data sets of the original documents and configured, when the other of the pair of the image data sets to be placed on one and the other sides of the common recording medium is to be formed as an image on a page preceding the one of the pair of image data sets, to allow the image forming section to form the image from the other image data set on an area from a leading edge to a middle of the recording medium in the direction of conveyance of the recording medium by the conveyance section.