14 awg-ish. Maybe 16. It isn't a large wire. I Also added an 8 awg ground from the cage (the bottom part of the shield you're bolting the stand-off to) to the body of the car for good measure. It might have helped smooth the idle a bit... or I might have imagined that.
A spacer would be easy to make with a short piece of tube and a longer 10mm bolt. Use it on the, easy to reach, lower right corner as it'll be a little hard to hold in place while you thread the bolt through. Stuff that can be found at Lowes/Home Depot kind of places.