1. A document original size detecting device comprising: a plurality of sensors each including a light emitting portion and a light receiving portion and respectively disposed as corresponding to different sizes of document originals to be placed on a transparent document platen, the light emitting portion and the light receiving portion being positioned above and below the transparent document platen, respectively; and a detecting section which determines a document original size based on outputs of the sensors; wherein the light emitting portion of each of the sensors emits light to illuminate at least two positions on a document placing region of the document platen on which a document original to be subjected to size detection is placed, and the detecting section determines the document original size based on whether or not the light receiving portion of each of the sensors detects the document original blocking the light in at least one of the two positions, and wherein the light emitting portion of each of the sensors includes at least two light emitting elements, and the light receiving portion of each of the sensors includes at least one light receiving element which receives light beams emitted from the light emitting elements.
2. A document original size detecting device as set forth in claim 1 , wherein the light emitting elements of each of the sensors are arranged so that the at least two positions on the document placing region are aligned parallel to a reference line along which one edge of the document original is positioned on the document platen.