There might be enough room between the piston and jack cylinder to either send down some 1/2" ridge pipe, small stiff copper tubing or maybe even a soft flexible tape with a weight. When either of these is bottomed out at the bottom of the cylinder you could pull it out and measure to get a length. You would of course have to land the car and pull the head off to see if you have the clearance between the piston.

This over travel situation can be dangerous, there was a guy at another company in my area that ordered a jack incorrectly and actually ran the car literally right through the roof when he went to do the inspection! There are some pics floating around online.