When replacing a selenium rectifier, how do I know what value resistor to use

[Electronic M]

There are various ways. If the set still works fine with it and replacing it is only a preemptive safety measure then you can measure the DC voltage drop on it in circuit and the current through it, and use those values to calculate the resistance using Ohm's law.
Another way (if you know the designed voltage drop) is to forgo the resistor and put modern diodes in series until the voltage drop is as close as possible to that of the selenium. Modern Si diodes have a drop of 0.7V and when in series those drops add up.

If the original is shot there is the guess and check method. Basically you want to use a variac to set the input line voltage to what the schematic calls for, check how much higher than spec your B+ is, and the B+ DC current draw. You can use that to calculate the number of series diodes or approximate value of the resistor (there are effects more complex than a single ohm's law calculation at play so the resistance value often does not yield the exact voltage drop desired).

If this is an AC/Battery radio with a 1L6 I recommend swapping that tube for a 1R5 or other cheap common type until you have the voltage on the heater string correct.