Alright...I sincerely appreciate the assistance:
2 full bathrooms-regular shower heads
2 people, 3 dogs
Wash boat and car at times
may run d/w and/or washing machine while showering
3/4 copper coming into house
City water, 6gpg
Looking at Aquapure softeners (Clack ws-1 cs head) see PDF
From a grain standpoint a 0.75cuft would suffice but probably need 1cuft for flow rate and possible growth in number of people (which would make it more likely that both showers would be running while someone is washing a car, etc).
ALSO....the aquapure guy suggested a sediment filter but I have not seen this as a standard suggestion....truly necessary with city water.......I have not noticed a significant sedimentary issue.
I would go with the 1 cubic foot.
Set it up with 3 lbs of salt/regeneration and 12000 grains capacity. 14 day override. Program P7. I would not install a prefilter--it is not necessary.