You are replacing a system that has been running for years, and the old unit's size is the first thing most people reach for. That is a starting point, not the answer. The old unit was sized for the house and the household as they were when it was installed. Your house may have changed since then.
Walk the house as it is now
Note rooms that were added, finished, or closed off. Note whether a basement or attic is now living space. Note windows that were replaced, insulation that was added, and any room that used to be storage and is now used every day. Each of these changes how much heating or cooling the house needs.
Then walk the household. Who sleeps where now? Who works from home? How often do two showers run in the same hour, or the laundry and the dishwasher at once? A water heater, a dryer, and a furnace each respond to a different kind of use. Write the household notes down by system.
Read what the old unit was asked to do
The old unit shows what it was asked to do, not what the house needs now. Look at the rating plate, the duct run it sat on, and how it cycled. A unit that ran long and steady on every cold morning was carrying more than a unit that ran short and stopped. Ask whether the old unit was ever upsized or downsized from the original plan. If a previous owner is available, ask which rooms they remember as hard to keep comfortable.
Ask for the reasoning behind the new size
Ask the installer for the load calculation used to choose the size. A written calculation shows its inputs: the floor area of each heated space, insulation levels, window area, air leakage, and how many people live in the house. Compare those inputs to the house you just walked. If the inputs describe a house you no longer live in, the size is wrong for your house, even if the calculation itself was done correctly.
Do not accept a size because it matches the old unit. Matching is easy to explain and hard to justify. Ask why this size was chosen and what would change the answer.
Fit the system to the rooms it has to reach
Sizing is not only about the total heating or cooling the house needs. It is also about whether the ducts or pipes can move that heat to each room. A unit sized for the whole house can still leave a back bedroom short if the duct run is long, undersized, or leaky. Ask how the ducts were checked, and whether any were changed as part of the job.
If you are weighing a heat pump, ask how it will handle the coldest days your area actually sees, and whether a backup heat source is part of the plan. If you are weighing a central air conditioner, ask how the airflow is balanced across the rooms you use most. The heat pump and central air conditioner pages list typical service life and planning dates, which help when two options with different lifespans are on the table.
Fit the system to how the house is used
A house that is empty most weekday afternoons needs a different plan than one with people home all day. Ask whether the system will be asked to bring a cold room back up after a long absence, or to run steady through a long stretch of heating or cooling. Ask how the controls handle any zones, and whether the thermostat sits in a room that matches the rooms with the most comfort complaints.
If the gas furnace is the system being replaced, the gas furnace page covers typical service life and when to plan for a change, which can be checked against the household notes you wrote down.
Put the questions in writing
Before you agree to anything, ask these questions and get the answers on the paper that describes the work. What load calculation was used, and what inputs did it assume? Which rooms were checked for airflow, and what was changed? How does the new size compare to the old unit, and why? What happens on the coldest day or the hottest afternoon the house sees? Which duct, vent, or return changes are included, and which are left out? What would need to change if the household grows or a finished space is added later?
Compare the paper, not the pitch
Lay two proposals side by side and look for the same load inputs, the same duct assumptions, and the same scope of work. A proposal that gives a size with no reasoning gives you little to check. A proposal that shows its reasoning gives you something to test against the house you walked.
Keep the reasoning with the equipment
File the load calculation, the room notes, and the household notes with the installation paperwork. When the system is replaced again, the person planning it will have the reasoning as well as the old size. Age is a planning input, and so is sizing: the house changes over time, so the record of why the system was sized the way it was should change with it. If you want to see how the other major systems in a home compare on typical service life, the six major appliances ranked by typical service life page is a useful place to start.