In an image scanning device, the receiving unit receives a light reflected on a document putted on the surface of the board. The presuming unit presumes a dimension of a horizontal edge of the document based on the reflected light. The determining unit determines, based on the presumed dimension, whether or not the document has one of the plurality of standard sizes. The assuming unit assumes that the document has a virtual size defined by a virtual horizontal edge and a virtual longitudinal edge if the determining unit determines that the document has neither the plurality of standard sizes. The virtual horizontal dimension of the virtual horizontal edge is equal to or larger than the presumed dimension. The scanning unit scans the document over a range corresponding to the virtual size to generate image data.