1. An image reading device comprising: an image reading portion which shines light on a predetermined reading region including a region where a document is present, and which reads the reading region; an image processing portion which generates image data of the reading region read by the image reading portion, and which detects a size of the document based on the image data of the reading region, the image processing portion performing image processing on the image data of the reading region based on the size of the document and outputting the processed image data; and a storage which stores size information that is information in which vertical and horizontal dimensions for each of a plurality of kinds of reference sizes are defined, wherein the image reading portion performs reading of the reading region with the document set in the reading region such that two edges of the document are aligned with reference lines that extend in main and sub scanning directions, the image processing portion generates, as the image data of the reading region, image data for output and image data for size detection, the image data for size detection having a lower resolution than the image data for output, the image processing portion discriminates, from the image data for size detection, high density lines, the high density lines being lines of pixels that appear as a result of a shadow cast in a vicinity of an edge of the document that does not overlap the reference lines being read, the high density lines extending in one direction out of the main and sub scanning directions, the image processing portion performs a target line setting process to set one of the high density lines as a target line, the image processing portion calculates as a width for size detection a width from, out of the reference lines that extend in the main and sub scanning directions, a reference line that extends in the one direction out of the main and sub scanning directions to the target line, the image processing portion sets as the size of the document a reference size having a width closest to the width for size detection out of the plurality of kinds of the reference sizes, and the image processing portion performs image processing on the image data for output based on the reference size set as the size of the document, and outputs the image data.