As Steve pointed out the size of the cables is dictated by the load (Amp draw) and the length of the cable .
Because we are working with DC ( Direct Current ) and low voltages the volt drop becomes an issue as the load increases or the length of cable increases.
The inverter / charger is a good example of a voltage sensitive device, if at high load the voltages are pulled down too low on the inverters input the inverters control board will shut it down as if the batteries were dead .
Inversely if the charger is trying to charge the batteries at a high rate the volt drop would cause a lower than expected charge voltage at the batteries.
As Steve pointed out there may be two different loads connected to the same battery bank each having its own load characteristics and requiring different cable sizing .
It is important to know what the cables are going to operate the Amp demand and the length of the cable run to determine the cable size.
The following link will help determine the size of the cables needed for a given length and load.
Remember here there is no such thing as too large a cable the bigger the better as it reduces volt drop.
Also remember that when calculating the length of a cable you have to calculate both directions to and from the battery bank.
https://www.bluesea.com/support/articles/Circuit_Protection/1437/Part_1%3A_Choosing_the_Correct_Wire_Size_for_a_DC_CircuitHope this helps
Eric