I measured the space I have available. There is a raised platform next to the water heater and space on the floor next to the pressure tank.
The raised platform is 17"x31" the area next to the pressure tank is 16"x31"
Height is not a problem.
Most retention tanks above 40 gallons appear to be at least 17" in diameter. So I may have trouble fitting a larger one in that area. I'm not sure if I would be able to fit both a carbon filter and a softener as well. The two tanks might fit, but the placement of the brine tank might be fun.