Using 5 watt solar panel for my FM200 Mighty Mule opener, what size battery do I use?

I am going to use a 5 watt solar panel for my Mighty Mule FM200 gate opener, the instructions read that I can use a car, lawn etc battery but it neglects to read what size battery to use. What is the best size to use for this install. I saw a 750 CCA in a video online but that seems overkill for a 5 watt solar panel.