The square foot of the building has nothing to do with sizing a softener etc.. Just the number of people in the family and the number of bathrooms and the type of fixtures in them.
You don't size a softener by the 32K or other K figures. It is the cuft of resin and what type of resin, that dictates the size of the resin tank and that dictates the control valve that can be used.
That's because ALL softeners, regardless of the brand name or the control valve brand name, have an adjustable K of capacity.
That's because they have an adjustable salt dose that is set by the number of lbs used per regeneration. It's like if I said regardless of the vehicle's fuel mileage figures, your right foot controls your fuel mileage.
SST-60 is not going to provide enough increased salt efficiency to justify it's high cost. So anyone attempting to sell it to you based on salt use does not have your best interests at heart.
I suggest you look at the Clack WS-1 control valve. It has many features a 5600 doesn't have. And IMO, based on good salt efficiency (not the constant SFR gpm for your peak demand, which I don't know yet) you need a 2.0 cuft softener.
To learn all about correctly sizing a softener click Softener Sizing and at the bottom of that page, after doing your math, click on Calculator and check your math.