Big = Bad
If any room in your house is more than 2-3 degrees different from another, there's a problem, and a furnace or AC that is too big is very likely a major part of the problem.
If you want to be comfy, the odds are good halving the size of your current HVAC is in your future. This guide carefully lays out why. Worried about freezing or burning up on really cold or hot days? It addresses that too!