Geoff,
I installed a Maxprop two seasons ago on my 32-2 and knowing that the pitch was adjustable, I kept the same diameter as the original.
My intention with to try and keep similar engine loads and operating RPM range.
Many years ago a family friend replaced a fixed prop with a folding prop. The diameter of the prop needed to be reduced so it would clear the E32-2 rudder when folded. However, the smaller prop seemed to need the engine to spin 'faster' and the life of that A4 was dramatically reduced.
Maybe something to consider.....
Mark