Calculate your total maximum potential amperage load first. This will tell you what size cable you need.
ECU, injectors, fans, pumps, stereo, headlights, horn, tailights, domelight, A/C, signals, markers, brake lights, 12v acc, oxygen sensors, ect. 

Make sure you pay attention to the ground capabilities also. You need low resistance on that side of the circuit as well.

Depending on your grounding scheme preference, nearly 45-50 ft of 1/0 cable is probably what is spec' for your application (charge+ground).

(I would keep it simple and use a modern lightweight battery up front. )