A size and position detecting system includes a laser sensor and a controller. The laser sensor includes an emitter emitting a laser beam to a receiver. The controller is configured to compute a first time of how long the emitter emits the laser beam and a second time of how long the receiver receives the laser beam when a cutting tool moves along a first direction perpendicular to and coplanar with a straight line connecting the emitter and the receiver. The controller is configured to record a first position of the cutting tool at a moment when the second time is less than the first time, and a second position of the cutting tool when the second time remains changeless for a predetermined amount of time, to determine whether the cutting tool is off-centered according coordinates difference between the first and second positions along the first direction.