I'd go with as big as you can afford and fit, you can never have too much battery power. The less you discharge a battery the longer it lasts, so if you can fit a group 31 in your compartment go for that.
I have 300 lbs of batteries in my boat
And at times, I'll use every bit of that juice over a fishing trip.
It's like having 4wd on your truck. You don't always need it, but when you do, you're sure glad you got it.