The larger gauge wire you use the less resistance you'll run into. I wouldn't relocate with anything smaller then 4 gauge, and I would seriously consider 2 or 0/1. Just make sure if you do relocate that you fuse it...don't want to turn your ride into a weenie roast.
One thing I've heard that people are doing is just getting a smaller battery (like miata size) then they are able to turn it side-ways and run their CAI beside it. Maybe this would work for you? Good luck with the mods.