Need help sizing zener diodes and varistor for 12v automotive power supply protection
Protection is against all you mentioned, plus Murphy. 

MOVs are fast but use up some lifetime every time they absorb energy.
MOV direct at source may have a brutal life in an autoenvironment. Or not. 
A little prior resistance may help lifetime. 

Zeners operated well inside specs have long lifetimkes.
Zener as shown dies-ish as no series R except wiring.
Work out 


worst case regulator Vin 
at max current out and 
worst case spec sheet dropout at that current.   
worst case LOWEST V12 that you want supply to hold up at (maybe 12V maybe less. 


Cranking may be hard and a large cap will help holdup. 



THEN a series R = Rs will drop
Vdrop = (Vin-MIN - V_Reg_In_MAX) = Vr.
Zener is set to hold Vin_Reg to MAX safe level less a bit.
Rs = Vr/reg_Imax.
Pzener = (Vinmax-Vzener) / Rs x Vz + a bit.
Vinmax is under fault or spike and is short term.
eg if you can have 2 x Vbat = say 24V for 1s and zener = 15V and Rs = SAY 10 Ohms (example only_
then Pz for 1s = Iz x Vz
= (Vin-Vz)/Rs x Vz
= (24-15)/10 x 15
= 1.35W. 

So eg a 2W zener will handle this easily and a 5W or 10W zener will handle MUCH larger peaks "for a while" . 

Power in Rs = (I_max_Rs )^2 x Rs = later ...

E&amp;OE