I think 4ga is the standard. 0 gauge is HUGE, and sort of a pain to work with, but I know I'll never have to upgrade it. Definitely go at least as large as your largest power wire - i.e. if you have 4 ga from the alternator to the battery, it would be pointless to have anything larger running to your components. Since I wanted 0 guage to my power distribution / fuse block, I had to go 0 gauge all the way to the source.