The 500 GPD... if that is your water meter reading, does it include irrigation? Average daily water use should be closer to 60 Gallons per person per day. Irrigation should not be fed off of the softener.
Assuming a properly designed system, and slightly lower water usage, a 2.5 Cu. Ft. system 7000SXT would be perfect for your application. The system size is also normal looking. 3 Cu. Ft. and 4 Cu. ft systems are huge in comparison.