An electronic device includes an image capturing device, a touch sensitive display, a distance measuring device, and a processor. The image capturing device picks up an image of the object and determining a focal length of the image capturing device at which the image capturing device captures the image. The touch sensitive display displays the image and receives input touches on the image to determine a touching region. The distance measuring device determines a distance between the electronic device and the object. The processor extracts the dimension of the touching region to determine the dimension of the image of the object, and determines the dimension of the object based on the dimension of the image of the object, the distance, and a pre-setting proportional relationship about the dimension of the image of the object, the distance between the electronic device and the object, and the focal length.