A method for measuring size of an object is provided. The method includes controlling a distance measurement unit to measure a vertical distance between an electronic device and the object in response to a measurement operation, controlling an image capturing unit to capture an image in front of the electronic device, which includes an image of the object in response to the measurement operation. Computing an actual size of the captured area according to the distance measured by the distance measurement unit and an angle of view of the image capturing unit. In addition, obtaining the image of the object from the captured image, and further computing the proportion of the image of the object in the captured image. Then computing the size of the object according to the proportion and the actual size of the captured area, and displaying the measured size of the object.