It won't run as efficiently as a properly sized unit.
My oversized 83% efficient furnace was only on 1/4th of the time in a typical winter, based on my gas bills and the input BTU/hr. We used maybe 300 therms in January and 10% of that was for the water heater.
Getting furnace efficiency data so you can do financial tradeoffs can be pretty difficult. It also depends on how much you pay per therm for gas or electricity. 1 therm = 100,000 BTU = 29.3 kwh.