Continually draining

Then measure it from the floor up to the bottom of the control valve where it screws into the tank, and then across.

The resin tank is the tank under the control valve. Resin is the media that removes select contaminants and has a cleansing stage(s) that removes those contaminants. The brine tank holds the salt.

Generally, the size of the tank will indicate the amount of media (depending on the media) that is inside.

The resin tank is approx. 44.5" to the valves and 28" circumference.

-gl

For those that may have misplaced their calculator, the diameter = circumference / PI = 9".

That is a 1 cu ft softener and will regenerate every 2-3 days with 7 people on that water with 0 iron. Make sure it is keeping proper time of day, if not the timer/drive motor needs replacing. If motor is bad it can be stuck in cycle. There may also be debris in the valve which can cause the piston to stick and run to drain. That system is too small capacity for your household. It may run every day if you are heavy water users.
Timer motor and piston are about $200.
RJ

WOW! $200 for a 5600 motor piston and a seal kit? He should buy online for less than half that including shipping.