12 gauge breeder??? sounds more like your shotgun ammo???
like trance said the lower the number the thicker the cable.
i have a battery that is half the size of the normal sx battery in my sx. havent had an issue for the last 3 years!!!
except that it really does move around quite a bit in that huge battery tray in the engine bay, especially when i'm on the track...
so i reckon the best would be a small little battery, relocated to the boot. no problems.
and if you were worried about the increase in resistance as the length of the cable increases, why dont you follow the flow of electrons from the battery to the alternator, to the supermulitplejunction, to the computer box, and then back to the engine. all in all, if you put all of the harnesses together, i guesstimate its probably about 10 or more meters long.
the increase in resistance should be negligable!