The only way to know if your furnace is correctly sized is for your HVAC contractor to do a load calculation. A load calculation takes into account the size of your house, the insulation value of your windows, the amount of insulation in your walls and roof, and a host of other criteria to determine the appropriate furnace capacity.

Ask your HVAC contractor if s/he performed a load calculation before you accept the furnace that recommend. If you are replacing an old furnace (as opposed to buying a furnace for a new home you are constructing) they may argue that they chose a furnace as big as your last furnace. But your old furnace might be too large for your house. Request that they perform a load calculation, and find another contractor if they refuse.