Furnaces are sized according to the cubic feet of the area being heated as well as the heat loss of that area. Providing that there are no demands out of the normal this is the only info you need.
You are going to need to look at the tag and see what it is capable of warming up to and how many square feet. Know the measurements of your home and then you can tell what furnace you need.
First, get a qualified person to do a heat load calculation on the dwelling to be heated. Without that, you will not have the necessary information to size the furnace.
