The size of furnace needed to heat a 2,000 square foot home is at minimum of 50,000 to 60,000 BTU in size to heat the 2,000 square foot home efficiently.
If you live in a colder climate then you might want to install a larger furnace so that it would be able to keep up properly with the colder climate.
If you live in a really cold climate I would go with a 72,000 to 100,000 BTU furnace so that you make sure you have plenty of heat.
Having too small of a furnace will cause the furnace to constantly run to try and bring the heat up to temperature.
It's better to have a bit larger furnace than you're required so that your house will heat up better and faster.