I can measure one for you, but, I don't know for sure how they are measured. When I ordered a set of 30 spline axles, I just told the guy that I wanted a set of axles for a G-body with a 8.5" rear and he knew what they were....that was Moser.
Moser sells standard replacement 28 spline axles for our cars. If the supplier you are calling is asking for length he is making you custom axles and probably a lot more money. Call Gears R Us they seem to have the best price.