A size inspection device includes an inspection frame, a transferring belt, a positioning assembly, an inspection assembly, a suction assembly, and a controller. The transferring belt is slidably mounted on the inspection frame. The positioning assembly, the inspection assembly, the suction assembly, and the controller are mounted on the inspection frame. The controller is electrically connected to the inspection assembly and the suction assembly. When the positioning assembly positions the workpiece on the transferring belt below the inspection assembly, the inspection assembly obtains size value of the workpiece and transfers it to the controller, the controller compares the size value to a preset standard range to determine whether or not the workpiece has passed quality inspection, and controls the suction assembly to handle the workpiece which is not of passing quality away from the transferring belt.