I've already said in the pneumatics thread it's 40mm stroke.

The seller isn't consistent in the strokes length...
NEW SMC PNEUMATIC CYLINDER

CDQSB16-40DC

MAX 1 Mpa (145 psi)

4 MM ID FEED HOLES

16 MM BORE

50 MM STROKE

72 X 29 X 29 MM

AND

PNEUMATIC CYL-SMC-CDQSB16-40DC

BORE16-STROKE 54MM-NEW wrong!

The 40 in the part number is the stroke length.