Johnny,
If you have 12 volt lights and 12 volt batteries, then you want to hook the batteries up in parrallel, sothe combination isstill 12 volts, but can supply twice the current of a single battery. Just hook the positives to each other, and the negatives to each other, and treat them as a single battery. I'd advise covering up one or both terminals since a short curcuit on a combo like that can have a very enlightening arc-flash.

About the wire size, that totally depends on the current you are pushing through them, and that depends on your load (lights). As a general rule, bigger wire is safer andwill run cooler. You can't get too big, only too small.Each of your lights will have a current rating (amps) or power rating (watts) listed along with the operating voltage. I (or many others here I'm sure)can help you run the numbers if you know the watts or amps.

Also, I'd recommend having a master fuse for the whole thingy, not just individual fuses at each light, and your master fuse should be sized accordingto your total load and corresponding wire size. Be aware that devices like switches also need to be rated for the power they will be used on, or they will overheat or melt. Again bigger is better.They may alsohave different ratings for AC vs DC applications. They can always handle more AC than DC, so read carfully or make sure that the guy helping you pick out your components knows the difference.

I do electrical stuff for a living, so feel free to post here or PM me with any other questions or specifics. The calcs aren't really tough at all. You just have to know when to apply which rules.

Good lord, I've written a book.
Good luck.
-Bulz