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there are factors such as exposure, windows, climate etc...that is why usually a manual J calc is done. But I can tell you that you are likely around 70-80 thousand btu....and also the duct size is important
wifi stats are more of a new toy. They save you nothing compared to a programmable with a standard schedule.
As far size of furnace...sq ft means nothing if the duct work is not big enough. You can have the best water pump there is...but if all you have is old 1/2 inch hose...thats all your getting.
You are burning 120,000 btu but wasting much of it. It would pay to identify 3-4 rooms you use consistently and improve the ductwork to them..even a couple rooms would be better than none.
Until you get the delivery system fixed the furnace will not matter.