8mm or 8B&S (about the same) is perfect. Just don't go and put in an in-line fuse holder that runs 4mm cable to spoil it all....

Grab one of these to match your cabling, then make sure the joins are done well - I used joining lugs and a proper crimper.

http://www.ebay.com.au/itm/MAXI-BLAD...0AAMXQgoBRheXp

6mm will work for a short run and you can get the same type of in-line fuse holders with 6mm cabling. 6mm auto cable is not the same as 6B&S though.

If you want to know more about it, there are some good articles on ExploreOz:

http://www.exploroz.com/Vehicle/Electrics/Solar.aspx