I would recommend a 2 Cu. ft. 7000SXT for maximum efficiency. It would be listed as a 64,000 grain unit, but its actual real world capacity would be closer to 48000 grains.
4 people x 65 gallons x 18 grains x 7 days minimum between regenerations = 32,760 grains. A 2 cu. ft. unit would regenerate about every 10-12 days.
Hope this helps!