Using the information calculated above, multiply your water usage (estimated or average) by your total compensated hardness. This will provide the average daily grains of hardness that you need to treat. Multiply that number by the number of days between regenereation desired to get your ideal grain capacity. With iron or manganese in the water 3-7 days is common (minimum of every 7 is recommended to prevent resin fouling). Water without iron or manganese are typically sized to go 7-14 days. Once you have your ideal grain capacity you can use it to find the system that most closely matches it.
NOTE ON SYSTEM CAPACITY: Water softeners are rated based on a maximum capacity using a high salt dosage. It is typically recommended to use a much lower (and more efficient) salt dosage, but this will also lower the available treatment capacity. If you want a more efficient system, most systems will operate at roughly 67% of stated capacity when using efficient salt settings (and use only 37% of the salt). You can use the "Efficient Grain Capacity" to see what rated size you will need to use efficient settings, or multiply the stated capacity of a system by 0.67 to get the efficient capacity.