First off, you need to figure out you amp loading.... how many amps does your furnace take, lights, radio etc. and how long do they run. That should give you your amp hr rating. Then match your battery requirement up with the right amount of amp hours. Not all batteries are created equal based on size. For example, a group 27 can range from 60-100 amp hrs.
Here is a good site for calculating battery requirements
I can only say I think you are a pretty straight up guy and it wasn't your post that I was talking about at all. I was just commenting on the way these discussions generally go and if I would have made the second paragraph in a new post the forum would have merged them automatically anyway so I just used this approach.