A furnace for 1,200 square foot house needs to be a minimum of 40,000 BTU but 50,000 BTU would be even better especially if you live in a really cold climate.
And then if the house is 1,500 square foot then you need to go up to a 55,000 BTU to 60,000 BTU furnace size.
And for a 2,500 square foot house you would need a 100,000 BTU furnace.
Too small of a furnace will take longer to heat the place up and may never get the house to the desired temperature and run all the time.
But a right sized furnace will run and heat the place to the desired temperature and keep the house warm and shut off when needed.