DE filter flow is typically 1 - 2 GPM per square foot of filter membrane. Design flow for most residential pools is one equivalent volume every 8 hours.

Thus:

14,000 gallons / (8 hours x 60 min/hr) = 29 GPM

and

29 GPM / 2 (GPM/sft) = ~15 sft minimum size . . . or 30 sft for a more conservative rating.

With DE filters, bigger is usually better. In California, with oversize DE filtration, you might be able to run the entire season without re-coating, BUT there's a complication. Most DE filters 'drop' the DE off the membranes anytime the pump turns off. And while there you can OPERATE a DE filter with a rather low flow, you can't usually RE-COAT the membrane at that low flow. So, you need to match the DE filter with either a VS or 2-speed pump that runs ALL the time. Otherwise, you're going to have to pick a DE filter that recoats itself well. I think maybe the Hayward Perflex does so, but I'm not sure. I know that the otherwise desirable StaRite System3 DE filters do not recoat well at low flow.

By the way, ONLY the DE System3 is desirable; the sand System3 is inferior to Pentair's own TR series, and the cartridge System3 is nothing short of awful.