Value of R in reverse voltage protection circuit based on PMOS
for the pmos Vgsmax=8V and the Vz=7.88max for the zener diode.

I wouldn't want to sail that close to 8 volts. I'd be thinking about a 6.2 volt zener. With a 6.2 volt zener (MMSZ5234B) the maximum voltage is 6.51 volts at a test current of 20 mA. And there's the main point; you select your resistor so that at the maximum supply voltage (say 14 volts), there won't be more than 20 mA flowing into the zener i.e. no chance of the voltage creeping up toward 8 volts.
In other words I'm applying a little bit of headroom to the situation. The resistor value will be: -
$$\dfrac{14 - 6.51}{20 mA} = 374.5 Ω$$
I'd choose a 390 Ω resistor. Then I'd double check the MOSFET data sheet so that its power dissipation won't be excessive given the lowest value load you might want to use.