You need to find centerline of the crank and then find the centerline of the journals. That should give you measurement in inches. That is the crank size.
One way is to use like a wood dowel that will fit in one of the journal center holes. Then measure from the center of the crank snout to the center of the dowel.
I think you would multiply crank measurement x 2 to figure piston travel.
OR
You could take it by any machine shop and they could tell you