Water softener choice and future decreases in water useage.

Our old water softener needs replaced. We are currently a family of 10 with lots of teenagers that use a lot of water. So,we are probably going to need a fairly large system. I have tested the water yet, but probably an 80,000 grain system. However, over the next several years, we will have kids graduating and leaving home. Would it be better to get a somewhat smaller system or go with what we need now? (The old system was installed by the previous owner and severely undersized.) What do you recommend?