On a private well, the pump, the pressure tank and your household demand together decide how much water a treatment unit has to handle. A unit that suits a city supply can still starve your fixtures on a well if nobody measured what the pump actually delivers. Sizing starts with the system you already have, not with the tank on a sales sheet.
Write down what the well and pump deliver
Find the pump model and its rated output, the pressure switch settings and the size of the pressure tank. Look for the well log or the installer's paperwork if you have it. If you do not know these numbers, ask a licensed well contractor to record them. A treatment provider should not size a unit around a guess about your pump.
Measure the demand you really have
Peak demand, not average demand, is what the unit has to keep up with. Think about the moments when the most fixtures run at once, such as a morning shower while the washing machine fills and the dishwasher drains. Count the fixtures in each group and note which ones run together. Then ask the provider to size against that combined flow, not against one faucet.
Check the pressure drop at the flow you need
Every filter or softener adds resistance. Ask for the pressure drop at the flow you actually need, not only the maximum flow printed on the tank. A unit that moves the water on paper but drops your pressure below what the shower needs will feel like a fault even when it is working as designed. Also ask whether the pressure drop figure was calculated at your system pressure or at a standard test pressure.
Plan for pump cycling
A well pump runs in cycles, so flow and pressure rise and fall as it starts and stops. Ask how the proposed unit behaves during those transitions and whether the pressure tank cushions them. Placement matters here. Ask the provider to explain, in writing, why the unit goes where it goes relative to the pressure tank and the water heater.
Fit questions before you agree
Confirm the space for the unit and for future cartridge or tank changes, since a unit you cannot reach is a unit you will not service. Check whether it needs a drain point for backwash or discharge, whether it needs a power outlet, and where the bypass valve will sit so the unit can be isolated for service. Ask for a site drawing with dimensions before anyone cuts pipe.
What to ask for in writing
Request a sizing calculation that lists the inputs: pump output, system pressure, peak fixture count, the flow used in the calculation and the pressure drop at that flow. If a provider cannot show those inputs, they have not sized the system for your home, and you should compare other proposals before you commit.