8. 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, wherein the light emitting portion of each of the sensors includes a light emitting element and an illumination position moving section which moves an illumination position at which a light beam emitted from the light emitting element is incident on the document platen to the at least two positions, the at least two positions being aligned parallel to a reference line along which one edge of the document original is positioned on the document platen, and wherein the light receiving portion of each of the sensors includes a light receiving element and a light receiving element moving section which moves the light receiving element according to movement of the illumination position on the document platen.
8. 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, wherein the light emitting portion of each of the sensors includes a light emitting element and an illumination position moving section which moves an illumination position at which a light beam emitted from the light emitting element is incident on the document platen to the at least two positions, the at least two positions being aligned parallel to a reference line along which one edge of the document original is positioned on the document platen, and wherein the light receiving portion of each of the sensors includes a light receiving element and a light receiving element moving section which moves the light receiving element according to movement of the illumination position on the document platen. 10. A document original size detecting device as set forth in claim 8 , wherein the illumination position moving section includes a first board mounted with the light emitting element and a first movement mechanism which moves the first board by driving means, wherein the light receiving element moving section includes a second board mounted with the light receiving element and a second movement mechanism which moves the second board by driving means.