There are many advantages to larger systems, and few disadvantages. Higher efficiencies, higher flow rates, higher quality water... Some may argue that a softener must regenerate at least once a week, yet I have never read that from any resin manufacturer. Most commercial companies recommend no less than every thirty days as long as water is flowing assuming no unusual water conditions are present. Iron, manganese, or other known foulants.
Even at 6 pounds per cu. ft., a 2 cu. ft. system would be calculated as follows.
20,000 grains per cu. ft. x 2 = 40,000 grains total capacity.
2 people x 60 gallons per person per day x 12 grains hardness = 1440 grains per day estimated usage.
40,000 / 1440 = 27 days between regenerations, - 1 days usage. That is about as often as my system regenerates, and has been doing for over a decade.
You could set the salt down to 4 pounds per cu. ft. and get the regeneration a little more frequent.
In my professional opinion, you will do just fine with a 2 cu. ft. 7000SXT unit. It would be my preference.