111 mg/L / 17.1 = (6.5) 7 grains per gallon hardness + 1 gpg for minimal iron = 8 gpg. An average water consumption estimate per person is between 50 to 75 gallons per day.
Using an average estimate of 65 gallons/day X 2 ppl X 8 gpg = 1,040 grains/day softening load.
For improved efficiency, a 1 cuft (32K grains total capacity) will deliver 21,000 usable grains per regeneration cycle when using 6 lbs salt, or 24K usable grains when using 8 lbs salt.
21,000 / 1040 = approx 20 days between regeneration cycles
24,000 / 1040 = ~ 23 days
A 1.5 cuft (48K grains total) will deliver 31,500 grains using 9 lbs salt, or 36K using 12 lbs salt.
31,500 / 1040 = ~30 days between regen cycles
36,000 / 1040 = ~34 days
As regeneration should occur at least on a monthly basis, assuming you do not require a higher than average service flow to supply high flow appliances or fixtures (ie: multiple shower heads), it would appear that a 1 cuft softener would be appropriate for your needs.
https://terrylove.com/forums/index.php?attachments/resin-chart-jpg.53316/
In reviewing this site, you will likely recognize both Fleck and Clack control valves are highly regarded and most recommended.
Buying online is often not recommended as the control valve is not the only component of a quality softener. While on-line dealers may specify a great control valve, other components such as resin, tanks, and the bottom screen will often be lower quality so as to reduce their price to undercut their competition. As shipping is usually included, the online dealers will also typically not include gravel under-bedding as gravel will add to the shipping weight and therefore increase their expense.
Both Clack and Fleck have altered their policies to primarily sell through local area dealers who understand water treatment and will properly support and service the quality equipment that they sell.
With regard to the control valve model, consideration should be given to the diameter of the plumbing service in your home. A Fleck 5600 SXT will be best suited for up to a 3/4" mainline whereas the newer 5800, 5810 and 5812 will be more appropriate for larger diameter plumbing.