An image reading device includes a plate on which a document to be scanned is placed, a cover that is movable between an open position and a closed position at which the cover contacts the document, a position detecting unit configured to detect a position of the cover, an imaging unit configured to emit light towards the plate and having an image sensor configured to detect light reflected by the document, and a control unit configured to determine a size of the document based on a detection result of the position detecting unit and a detection result of the image sensor.