Hello @d.zelosko,
The selection of bias resistor depends on the COMP pin current or opto-transistor current, CTR(Current transfer ratio) of the optocoupler, Vout and reference voltage. By calculating the current through the FB pin and opto-diode via CTR, the value of the bias resistor can be determined. Usually, the bias resistor is around 10-50k ohm.
Hope this helps.