How to estimate the right size furnace for my house
You cannot estimate a furnace by the size of your house.   That is just a factor in the equation, and it is not 80% of the equation or even half.
So first when you have the size of your house you should not calculate square feet, it should be cubic feet.   A house with 8 foot ceilings is considerably smaller than ones with 10-12 feet ceilings.   If a furnace tech quotes you on the square feet in your house - wow get a new tech.
Also you need to factor in the cubic area of the unfinished areas that are connected - your basement, useable attic space, an enclosed patio.
Then you have to guestimate - there are services that will get this done for you accurately - how efficient you are insulating your house.   A house that is well insulated may require half the heating of a house with little to no insulation and bad windows/doors.
The last thing is the one-offs.   Are your duct runs longer than average?   Do your ducts travel in the attic or underneath foundation?   Do you have more ducts than normal for a house of your size?
You are rating the entire house system plus size.   There is no easy equation.   An old exterior door with gaps in the middle of the house could cause 10% heat loss.   The problem is if you rate too low your house will never be warm and you will stress the furnace and blower and it won't last as long.   Yes rating too high and you lose a little money in efficiency.   But really you always have to be a little over when estimating or willing to spend a lot of money insulating and replacing windows/doors.