Online sizing calculators are pretty good sometimes. Gary has a calculator on his site that is better than most. The most common factors that need to be considered are hardness in GPG, compensated hardness for Iron and excessive TDS, how many people in the house, fixture counts, pipe size, etc.
Most companies tend to go with slightly larger equipment the necessary to ensure the system is adequate for the application and future growth. The "48000" grain system is more realistically a 32000 grain system. Systems should be sized to regenerate no more than every 6th day for single tank systems. Anything beyond 8-10 days nets very little gain in efficiency but it does not hurt a system to regenerate infrequently. Every 30 days should be the max (no real reason why, just common practice), and most systems are programmed with an automatic overide to force a regeneration if the system does not exceed its capacity by that time. Old style electromechanical systems will sit indefinitely between regenerations if the meter does not zero out.
If you have iron, more frequent regenerations are preferred to lessen the fouling of the resin bed.
Hope this helps.