A surgical instrument is configured for determining the size and/or height of an intervertebral implant insertable into an intervertebral space between two adjacent vertebral bodies of two human or animal vertebrae. The implant includes a first abutment element for abutment against an articular surface of one of the two adjacent vertebral bodies and a second abutment element supported directly or indirectly on the first abutment element for abutment against an articular surface of the other of the two adjacent vertebral bodies. The instrument includes a holding part defining a longitudinal direction and at least one test implant, which includes a first and a second test-implant abutment element each having at least one abutment surface for abutment against one of the articular surfaces. A distal end of the holding part includes at least one abutment-element holding element for detachable connection to the at least one test implant 149.