In an image-reading device, when the reading unit generates the set of image data, the top side is generated at first and the bottom side is generated subsequently to the top side. The scanned image includes a document image corresponding to the document. The specifying unit, when the reading unit scans and reads a part of the document and generates a part of the set of image data including a part of the document image, specifies a target vertex that is positioned at topmost among at least one vertex of the part of document image. The estimating unit defines a reference position in the scanned image corresponding to a center of the conveying path in the width direction, and estimates a size of the document based on a length in the second direction between the target vertex and the reference position.