Quick take
64K can be appropriate for four-to-six people with very hard water or high demand. It can be unnecessary for one or two people with moderate hardness.
Calculate before deciding
Use gallons per day × hardness gpg to find daily grain load, then compare the target regeneration interval with usable resin capacity.
Signs 64K may be too large
Very low household use, moderate hardness and regeneration intervals stretching far beyond what is needed can indicate you paid for more resin than the application requires.
Signs 64K may be right
High hardness, larger households, iron compensation and high simultaneous flow can all justify the larger size.
Do not downsize based on price alone
An undersized system that regenerates constantly can consume more salt and water over time than a correctly sized larger unit.