Take two measurements.
1. The diameter across the skirt about half an inch up from the bottom.
2. The distance from the wrist pin hole to the crown of the piston.
It will be nigh on impossible to get one end of the caliper centered on the wrist pin hole. Do this,
Measure from the very top of the wrist pin hole to the crown and add 0.4635" to your measurement.
This should tell you what combination of stroke/rod length/bore size the piston is for.