Beacuase of the 2-1/5 bath flow rate potential requirements, I would recommend a 1.5 cu. ft, but regenerate it with a low salt setting. This will also allow for future family growth should that occur. Regenrating past 10 days gains very little efficiency, but it will not hurt the system either.
1.5 cu. ft systems also cost almost the same as a 3/4 or 1 cu. ft. system.
1 person x 75 gallons per day (single person tends to use more than 60, the only time I use 75 GPPPD is with 1 person) x 20 grains, = 1500 grains removal per day. At 4 pounds of salt per cu. ft, your system should regenerate approximately every 15-20 days.
The 5600SXT is perfect for your application, unless you have a mega shower.