A central cooling system is sized for the house as a whole, but it is controlled from one point. The fit question is whether that one point represents the rooms you live in. In a house with an upper floor, that is often where the trouble starts.
Start with the rooms you use
Walk the house in the afternoon, when the sun has been on the west and south walls for a while. Note which rooms are hot, which stay comfortable, and which you avoid in summer. Write the list down. The rooms you use most are the ones the system has to serve well.
Check where the thermostat sits
Find the thermostat and note its location relative to the stairwell, the hall, and the rooms with the most glass. A thermostat on a lower floor often reads the cooler air that settles there, while the upper rooms stay warmer. A thermostat near a lamp or a sunny window can also read high and cut the run short.
Ask for a room by room estimate
Ask the installer or the reviewer to estimate the cooling load for each main room, not only for the house. Ask for the estimate in writing, with the assumptions listed. Ask how the estimate treats windows, insulation in the upper floor, and any rooms that are closed off in summer. If the answer is only a single figure for the whole house, ask what it would look like room by room.
Ask what a zone or second thermostat would add
Ask whether zoning, a second thermostat, or a separate small unit is part of the plan, and what each option changes in the way the system runs. Ask how the air is moved to the upper rooms and whether the return air path is adequate. Ask what is left out of the plan, so you know where the gaps are before the system is installed.
Check the current setup
If the system is already in place, note the age of the unit and any change in how it runs. The Central Air Conditioner: typical service life and when to plan page gives the planning frame. Use that frame to decide whether a fit change should be planned now or folded into a future replacement.
What to write down
Keep a short list: the rooms you use, the rooms that run warm, the thermostat location, the room by room estimate, and the questions still open. That list is the sizing record. It stays useful even if the installer changes.