An image processing device includes a storage module, character recognition module, a circumscribed rectangle extraction module, a ratio extraction module, and a character size calculation module. The storage module 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. The character recognition module recognizes a character image in an image to get a character identification code from the recognized character image. The circumscribed rectangle extraction module extracts a circumscribed rectangle of the character image. The ratio extraction module extracts the reference ratio corresponding to the reference character identification code stored in the storage module based on the character identification code. The character size calculation module calculates a character size of the character image.