How to measure for furnace size

What is it that you are looking for in size. Do you want to know the BTU, or the AC capacity, or the capacity of air flow? CFM. Or do you need the electric power needed to operate it such as single phase or three phase.
To get any of these answers you need to have some numbers, The Model number in most cases will supply you with this information, Send me some numbers and I will try to decipher them for you.