How much it extends is less important than its minimum length. This is a case of the shortest distance being a straight line, in this case the straight line being when the shaft is perfectly parallel to the ground. At any angle above or below that, the shaft will extend. So the parallel length is the critical length.
With no shaft installed, raise the cutter so that the PTO splines are level with each other. If in doubt, measure the distance from the tractor spline to the ground, then adjust the mower height so the transmission spline is at the same height. Then measure the distance between the rounded grooves that go around the circumference of the spline (they're the channels into which the shaft locks). You need a shaft that will collapse to that length without binding. I seldom find one "off the shelf" that fits properly. Typically I already know the groove to groove distance, get one just a little longer than I need, then cut it down to fit. Also typically, a new shaft should come with instructions on how to cut it down to fit your application. Simple job; hacksaw, tape measure, and vise. The hardest part of the whole job is just getting the PVC shields off and back on.
//greg//
