1. An image processing device comprising: a storage module that stores a reference ratio between a reference size of a reference circumscribed rectangle and a reference character size in a reference character image representing a reference character in association with a reference character identification code which uniquely identified the reference character; a character recognition module that recognizes a character image in an image to get a character identification code from the recognized character image; a circumscribed rectangle extraction module that extracts a circumscribed rectangle of the character image; a ratio extraction module that extracts the reference ratio corresponding to the reference character identification code stored in the storage module based on the character identification code; and a character size calculation module that calculates a character size of the character image based on a size of the circumscribed rectangle and the reference ratio extracted by the ratio extraction module.