I would buy the biggest heat pump you can get. My pool is only 7K gallons, but I have a 144K btu heat pump. It heats the pool up in the morning this time of year to 88 in about 1-2 hours max, then doesn't run the rest of the day. During the colder months, it's able to keep up with no issues.
A bigger heater only costs a few hundred more, and because it runs less, will last longer. Larger heat pumps tend to have higher COP (efficiency rating), so also cost less to run. Also, if you are using a variable speed pump, you only need to run the pump at 75%-80% during heating, then can drop down to 35%-40% the rest of the day. Running a VSP pump at 40% vs 100% saves over 80% in electricity costs.
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