Choose the cables with the biggest cross section (biggest diameter) ; those will have the least resistance and allow a good flow of current.

I once tried to start a car with a cheap set of cables : the car wouldn't start ; too much loss in those tiny cables (high amp + high cable resistance >> huge voltage loss).
When tring with sturdier cables, the car started without problem.

And yes, those bigger cables are more expensive.

As for the length : longer cables >> higher resistance. If they already have a tiny cross section >> they wont do any good and can even be dangerous...

(excerpt from Electricity 101) <img src="/images/graemlins/grin.gif" alt="" />