Take a look at this
http://www.powerfilmsolar.com/It has enough power to be useful charging all kinds of portable electronics.
Yea it’s $500.00, but it's useable and has enough amperage to really charge something.
Small, low priced chargers look great as far as price, but they are lacking in real power and ability to charge things.
I would go with at least 20-watts. 20 watts is about 1.6 amps at 12-volts. But most solar panels have an open voltage (no load) of 20-volts. This makes them look better on the spec-sheet, but once you hook them up to something the voltage drops down to 12-volts (for a 12-volt panel.
I would get a 12-volt panel only. I have seen solar panels that are rated at 4 to 6-volts for cell phones. This really limits your options to use it for other items. The 12-panel will allow you the most charging options.