From experience I like 2/0 cables, even for 12 volts. They may be extra big, but they are still easy to work with and install, and do a great job.
It's also a great idea to run your battery ground cable directly to a mounting bolt on the starter, many times that solves a slow cranking problem. Most diesel engine starters have a ground post on the starter for connecting the battery cable to.
Another consideration is what happens when strands start breaking or corroding inside the cable, then bigger is better, as it will last longer.
When soldering cable ends on I like to use the heat-shrink stuff with the glue inside to help seal the cable ends from corrosion.
Another thing many people don't understand is how terrible those quick clamp on battery ends are, the ones with the sheet metal strap and two bolts that sandwich the cable to the clamp. given a little time they turn into really bad connections.
Another important tip, don't use acid core solder, or acid as a flux, when soldering electrical connections.
Grigg
