Quick take
Size the treatment train to the lower of two realities: household peak demand and sustainable well-system output. Then make sure backwash requirements are still met.
Pump GPM and well yield are different
A pump may be capable of a high instantaneous flow while the well itself recovers more slowly. Long-duration demand should respect both.
Pressure tank smooths cycles, not unlimited demand
The pressure tank stores a limited amount of drawdown and reduces pump cycling. It does not permanently add GPM to an undersized pump or low-yield well.
Backwash can be the highest single demand
A large media filter may require several gallons per minute of sustained backwash. Check that requirement before moving to a larger tank.
Install treatment after the pressure tank
Mainstream whole-house treatment manuals commonly place well-water filtration after the pump and pressure tank so the treatment sees stable pressurized flow.