This depends on the engine size and type of vehicle (petrol or diesel). The thicker the cables are, the more effective they will be to conduct the power from one side to the other. Please use the recommended table below:

Cable                  DIESEL       PETROL
25mm² 3,5M       3.0L           5.5L
35mm² 4,5M       4.0L           7.5L

IMPORTANT: You should always consider using a thicker cable if you have a doubt. Using a thicker cable for a small vehicle will not cause a problem but using a too thin cable for your vehicle can lead to overheating the cables