what size would you use...

I use #2, that way if I want to jump start it I know there is enough cable there to carry the load.

Depends on the length of the wire.

I used left over #2 that I had and the length is only about 2 feet on mine.

I typically use whatever size battery cable is being used on the rest of the car for the reason stated above about jump starting the car with a dead battery. I have seen people get away with as small as 10 gauge when weight is a concern and and the lugs will only be used for trickle charging. To me it seems like an impractical way to save weight.