The volume of the resin determines two things, how much total water it can soften and how fast it can do it. The larger the amount or resin, the more surface area, and the more water it can soften at one time as it passes through. So, you must have enough resin generated and capable of softening the water for your maximum volume at any one time. So your peak volume (not total) determines the maximum size of the softener. Now, since you don't want to regenerate that every day, it must also be large enough to handle your average need over ideally 7-10 days or so. The resin only needs enough salt to regenerate what was used, and the larger the unit and the lighter it is used in total, the less salt per quantity of resin needed. So, the amount of salt you need at a regeneration is determined not by the overall size of the softener, but by how much of its capacity you've used. So, if you've only used say 1/2 of the total capacity when it comes time to regenerate (based both on the number of days and the total voluem used), you only need enough salt to regenerate 1/2 of the tank, not the whole thing. Where you might need X pounds to regenerate say a 60K tank IF it was totally depleated, if you've only used up 1/2 of its capacity, then you only need 1/2 of the maximum amount of salt, but in the process, because it has more surface area, you could use more water at one time without having hard water bleed through. You can only soften so many gpm with a certain sized tank so the size needs to take into consideration max instant use AND enough to cover the average useage to cover you for at least a week or so or it starts to get inefficient or invokes more wear on the media.