The bigger the better. You can get the braided ground straps at most auto parts stores. Those work very well. And don't just look at a connection or tighten with a wrench. Actually take it apart, wire brush all the terminals and even use a little sand paper to clean up the metal around the bolt hole then put all the terminals back. And at the same time, you can check the ends of the wires for corrosion. They will often feel different than the rest of the cable, either very stiff or very flexible. You often get corrosion up under the insulation.