What size sand filter is needed for a 24-foot round above ground pool?

I think you should go for a cartridge filter (Pentair 320 sq ft - we can supply for $600 incl frt). Cartridge filters are much better than they used to be, and are far, far more coinvenient than sand. Suitable sand filter would be approx 100 sq ft (and same cost as 320 sq ft cartridge).