First welcome aboard!
Get the biggest one that will fit in your tray.Is this battery just for the engine or is there 12c accessories that need juice? Do you have a frig ? That would be the biggest consumption of power other than your radio. If you have said yes to these questions I would look into a deep cycle group 27 or 29. Another scenario is two batteries. One for the engine, a starting battery and the other, the house battery, for your 12v accessories which would be a deep cycle battery. Lots of options for different needs. So yeah what you have now is the very minimum for starting your engine and that's about it!