A water softener is sized around the people and fixtures it will serve over the next several years, not only the people in the house on the day the seller visits. If your household is likely to change, the size you pick now may be wrong well before the warranty runs out.
Write down who lives there now and who might
Start with a plain list of adults, children, and anyone who stays regularly. Then add the changes you can reasonably expect. An adult child moving back home, a new baby, a partner moving in, or a relative who will need more help with bathing all change daily water use. Ask the seller to size the unit for the household you describe, and ask them to state the household size they used.
Estimate use from your own bills
Do not accept a seller's guess about how much water your home uses. Your water utility bill shows total usage over a billing period. Divide that total by the number of days and by the number of people to get a rough daily use per person, then adjust for the change you expect. If you have a water meter, read it at the same time each day for a week to see the real pattern. Ask whether the sizing worksheet was based on your bills or on a generic per-person figure.
Check the hardness you are actually treating
Household size only matters in combination with water hardness. Ask for a written water test result, ideally from a lab, and ask how the seller converted that result into a daily hardness load for your home. If the test is missing, or the seller cannot show the calculation, the size recommendation is not yet verified.
Ask how the unit handles a heavy day
Two households with the same daily total can have very different peak days. Ask how the unit regenerates, how often it will regenerate at your projected use, and what happens when a guest weekend or a big laundry day pushes demand above the normal pattern. Ask whether the regeneration schedule can be changed without a service call, and whether the control shows how much capacity remains.
Compare a larger unit against a planned upgrade
Some households do better with a unit sized for the future. Others do better with the right size now and a clear plan to upgrade later. Compare the purchase cost of each option, the salt use at your actual regeneration frequency, the space the tank and brine tank will need, and what the warranty covers if your household changes sooner than expected. Ask each provider to put these figures in writing so you compare the same assumptions.
What to get in writing before you sign
Confirm the household size used in the sizing, the water test result and the hardness figure behind it, the expected daily use and regeneration frequency, the unit's rated capacity and its usable capacity for your water, and the terms for resizing if your household changes. Ask who will explain these numbers to you at installation, not only at the sale.
For the purchase steps in order, see The Missouri Water Softener Buyer's Guide.