There is a lot to sizing a softener, but with your application, a standard, slightly larger system will work fine.
More modern calculations are as follows.
6 people x 60 GPD (probably less but this is the new standard) x 25 grains = 9000 grains daily, x 7 days between regenerations minimum, =63,000 grains. A 2.5 cu. ft softener with a 1-1/4" control valve is realistically rated for 60,000 grains when it is regenerated with 8 pounds of salt per cu. ft., the maximum most companies recommend to maintain good efficiency for both water and salt. This is technically not proper for California and many other municipalities have followed Californias efficiency standards. This is where it gets difficult to meet the standards, reasonable costs, etc... In all reality, you will do just fine as I have stated. You could use a slightly larger unit, but the jump in equipment size is considerable, as is the cost. Peak flow rate of this system is in excess of 18 GPM, as is the "service flow". Consider your water usage, even with 6 people, will you ever exceed 10 or even 15 GPM? Not likely.