Question about diode strings in high voltage applications
The purpose of the resistors across the diodes is a balance the magnitude of the reverse voltage across each individual diode. 

A quick way to estimate the resistor value is to check the diode datasheet for the maximum reverse current at the maximum voltage you will be applying across the diode. Then choose a resistor value that gives about 10 times that current, again at the maximum voltage across the diode. 

You also need to ensure that the voltage rating of the resistor is adequate. That is: the maximum voltage across the resistor must be less than what the datasheet for the resistor says.