Most often, the larger cell doesn't cost more in direct proportion to the total chlorine it can produce, so a larger SWG produces total chlorine for less $ per pound.
With a large cell, you need enough % settings to fine tune it for a lower pool volume. Most seem to have eight or ten settings (12.5% or 10% increments). Even that many might not be enough with a 2.0 lb cell on a 7000 gallon pool, so I personally wouldn't go above 1.5 lb/day, which would produce 1 ppm FC per hour in 7000 gals. Less production per hour will certainly get the job done. A large Hayward could be awkward because of it's 3-hour cycle time. Their SWGs apply the % to the beginning of each run cycle, so for example, with 2 hours pump run time: chlorinator set at 50%; 50% of 180 minutes = 90 minutes; so the chlorinator actually runs 75% of the time (90 minutes out of 120 minutes). You can defeat the issue by using 3-hour pump run time increments.
The most frequently recommended brands here are Pentair and Hayward. There are also recommendations for CircuPool which sound like better value, with longer warranty and no self-install warranty penalty. Pentair replacement cells tend to be a few more dollars, probably because the flow switch is built into the cell.
