The size panel you should get depends on what you expect to use it for. If you are going to power a Dometic type refrigerator, 90 watts is about the minimum I recommend. If you will not be running a fridge, you should question if you really need a panel at all. We have been off the grid for as much as six days (no fridge) and still had plenty of battery left, using lights, water pump, furnace, and some TV. YMMV.
There are several threads on the forum here about solar panels. Do a search, there is lots of information here.