A size estimation method is used with a document camera to estimate an object size. The document camera includes a lens and an image sensor. The size estimation method includes the following steps. Firstly, an object distance between the lens and the object is detected. According to the object distance and a viewable angle of the lens, a shooting zone size is estimated. Then, the object is shot by the lens and the image sensor, thereby acquiring a digital image including an image of the object. Then, an effective pixel number corresponding to the object image is counted. Afterward, the size of the object is estimated according to the effective pixel number, a total pixel number of the image sensor and the shooting zone size.