You need to use a wire gauge calculator (example:
https://baymarinesupply.com/calculator ...just one calculator of many I randomly pulled up), to figure out what your gauge should be based on voltages, amperages, conductor type, and distance. You will need to know the PV run voltage and amperage (by calculating series/parallel number of panels, panel specs, to get PV circuit volts and amps max on the run), for the PV input side.
For controller output side (to battery bank), need to know the same, max amps and volts the charger can provide to the battery bank, and length, etc..
In many cases, a 10 awg wire is max PV input wire size (hard to find MC4 connectors that support larger than 10 awg), although I am running 8 awg on mine for long distance part of the run (and 10 awg from panels to the DC combiner box). Again, I added up all my panels, 4s6p and calculated max volts (Voc), and amps (Vmp) on the circuits, and sized accordingly to provide minimal voltage drop. I actually have 3 home runs, two 4s paralleled on each PV circuit for mine, going to 3 PV inputs on the charge controller end.
Battery side can depend a lot on whether is 12v, 24v, or 48v (as 12v has much higher amperage, than 48v for example), so lower voltage needs larger wires for example. The charge controller manual should be able to provide specs on its battery charging capability on the different voltages (and if it can operate on different battery voltages even).
And then you need to size breakers or fuses on both circuits in accordance to protect the wires.