25mm will be ok for smaller engined cars with little or no major electrical power draws...usually good for around 80-100amps
35mm will be ok for medium sized engine cars and audio systems/electrical current draw upto around 175amps
50mm will be ok for large engines, big audio systems, heavy electrical power draw upto approx 300amps

Also, DO NOT buy CCA cable especially for this application, you want OFC cable. Carries more current, more resistant to heat changes and doesnt break down once it has been compressed or exposed to a bit of water.

Welding cable is ok, but the cable sizes never match the cables that I use...for example, 50mm cable that I use is more like 70mm welding cable...but its MUCH more flexible making it easier to route through the car.