I would recommend at least 10 -20 watts . Have a look in SuperCheap Auto - they usually have cheaper ones.
For 12 volt lighting in my shack down the back paddock, I have a 20 watt panel with built in regulator charging a 12 volt battery.
For 12 volt lighting in my shack down the back paddock, I have a 20 watt panel with built in regulator charging a 12 volt battery.