A water softener is sized by how much hardness it has to remove between regenerations, and how much water your household uses decides how much that is. Two houses of the same size can need very different units. A family of five with two teenagers showering every morning uses far more than a retired couple who leave for weeks each winter. Start with the people and their habits, then compare equipment.
Count the people who actually live there
Write down everyone who uses the water every day, then add anyone who stays often: adult children home for long stretches, a relative who visits for weeks, or a home business that uses water during the day. Count the bathrooms in regular use, not just the bathrooms on the floor plan. Laundry and dishwashers matter, and so does any outdoor hose or irrigation that runs off the same line.
Know your hardness before anyone sizes the unit
The hardness of your water sets how much work the softener does. Look for the figure on your water report, or get a lab test if you have none. A provider who sizes a softener without a hardness number is guessing, so ask what figure they used and where it came from.
Household patterns that change the answer
A home that sits empty for part of the year uses less water, and the softener may regenerate on a schedule that does not match real use. Ask the provider whether the unit regenerates on a timer or based on how much water has been used, and which one suits your pattern. A household with heavy evening use needs a different plan than one that uses most of its water in the morning.
If you have a well, ask whether the pump and pressure tank can keep up with the softener's regeneration cycle. A unit that works well on paper can still struggle if the water supply is slow to refill it.
Compare the sizing answer across quotes
Ask each provider to state the household size, the daily water figure, the hardness figure, and the regeneration method they assumed. If two quotes assume different household sizes, the cheaper one may simply be undersized. Ask the provider to show the calculation, and check that the numbers match what you wrote down about your household. A sizing answer you cannot explain is not one you should pay for.