Little confused
When you ask size, do you mean capacity in terms of amp/hours? I'm going to assume yes. If you have the space for it I would recommend a group 27 deep cycle, especially if you are going to be spending nights onboard. If you don't have the space then a group 24 should also be sufficient. I can't imagine an O'Day 20 having any 24 volt systems onboard. Nor have I ever heard of any 6 volt sytems. So I should think it is safe to assume it is a 12 volt system.
