For the wires, the bigger the better IMHO. Smaller may be easier to run through things, but bigger will carry more current more reliably with less drop.
Though realistically, the amount of current needed will influence the wire size needed, and will determine the fuse size. So you need to figure the current draw of all of the stuff you want to plug in, and then pick a fuse size somewhat larger than that, and use that larger fuse size (amperage) and the length of the run to select the wire size.
Or just guess...
-soD