I've never used any lighting accessories but I'm sure BionX used one of the industry standard sizes. I don't have a micrometer but I'm going to guess it's the Type A, 5.5 mm OD with 2.1 mm center pin. That said it might be a Type B with 6.0 mm OD, 2.1 center pin... it's hard to eyeball precisely with a ruler. The male plug from my scanner power supply fits exactly. The sleeve is 9-10 mm long.
IEC 60130-10:1971 defines five DC power connectors.
Type A: 5.5 mm OD, 2.1 mm ID (with optional screw lock)
Type A: 5.5 mm OD, 2.5 mm ID (with optional screw lock)
Type B: 6.0 mm OD, 2.1 mm ID
Type B: 6.0 mm OD, 2.5 mm ID
Type C: 3.8 mm OD, 1.4 mm ID
Type D: 6.3 mm OD, 3.1 mm ID
Type E: 3.4 mm OD, 1.3 mm ID