Panel sizing
The panel size you need depends upon the size of the battery bank you want to keep charged. For trickle charging -- just keeping your battery topped off, you want you panel to put out about .5% of your batt's amp hours. So if you have a 200 amp hour bank, you want about a 1 amp trickle charge which is about 15 watts. (15 watts / 15 volts ~ 1 amp)If you want the solar to recharge your batteries, like the others say get the biggest you can, but you'll also need to wire in a voltage regular so they don't fry your batteries. Smaller regulators are not expensive, can be found for $20-40.