A sheet size detecting mechanism includes a sheet guiding module, a sensing module and a control unit. The sensing module includes a contact element and a circuit board. The circuit board includes a first conducting part, a second conducting part and plural fixed resistors. The first conducting part includes plural conductive segments. The plural conductive segments are connected with the corresponding fixed resistors, respectively. While the sheet guiding module is moved relative to the sheet input tray, the contact element is moved with the sheet guiding module. When the electric connection between a first conductive segment and the second conducting part is established through the contact element, the corresponding fixed resistor provides a resistance value, and the circuit board issues a sensing signal according to the resistance value. The control unit acquires the size of the paper sheet according to the sensing signal.