An instrument for use in determining an anterior-posterior dimension of a femur during a minimally invasive surgical procedure includes a body having a base and a member extending upwardly from the base, said member providing a first part of a sliding mechanism. A stylus includes a stylus support providing a second part of a sliding mechanism which can engage with the member to allow relative sliding movement between the stylus and base, and a stylus member mounted on the stylus support. An implant size indicator interacts with the sliding mechanism to be moved relative to a scale by the member to indicate a size of an implant corresponding to the relative positions of the body and stylus.
