What are the correct zener diode and PTC fuse size?
PTC fuses can be wonderful in some ways, but they're not very good at providing overvoltage protection for Zener-clamp circuits.  Suppose, for example, that one has a Zener that clamps absolutely rigidly at precisely 30.0 volts, the PTC has a tripped power dissipation of 7W (grabbed from the data sheet of a 32V 10A device), and the supply feeds out exactly 30.5 volts.  Under that scenario, the resistance of the PTC would increase to 0.035 ohms and remain there indefinitely, passing 14 amps.  In such a state, the PTC would continuously dissipate 7 watts perfectly happily, but the 30-amp Zener would need to dissipate 420 watts--not just briefly, but indefinitely.  Note that if the supply voltage were to increase, the current through the Zener (and the power it would need to dissipate) would drop considerably, but in many scenarios a low-impedance supply is just as likely to be slightly over the required voltage as to be massively over.

If you want to use a PTC with a shunt, and are only interested in surviving over-voltage conditions rather than being able to operate through them, I would suggest using a circuit that selectively shorts to ground the voltage after the PTC.  When such a circuit trips, it will cause a lot of power to be dissipated in the PTC, and relatively little in the protection circuit.  Circuit operation will not be possible in such a circumstance, but downstream devices will be usable once everything is shut down and the PTC is allowed to reset.