A sheet size specification system for specifying size of a sheet (S) on a sheet stacking tray (21) of an image forming device (100), by using an image capture unit (203) of a portable terminal (200). The system includes an analyzer (51; 201) and a size specifier (51; 201). The analyzer analyzes image data captured by the image capture unit, the image data indicating the sheet stacking tray and the sheet thereon. The size specifier acquires, from the analyzed image data, first position data indicating positions of predefined feature points of the sheet stacking tray and second position data indicating positions of an outline of the sheet, and specifies size of the sheet on the sheet stacking tray based on the first position data and the second position data.
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