I'm assuming it is 630 cubic feet what would be roughly 17.9K gallons.
The cartridge size is the bigger the better, so you don't have to clean it often. Of course, if 400sqft (2x 200sqft) is what fits than there is not much to choose from.
I remember chatting with someone else from San Diego area... isn't your climate very dry?
Heat pumps work by extracting heat from the air and transferring it to the water. Dry air holds less heat than humid air (the whole reason why deserts are cold at night).
Assuming
this is your heat pump.... It has this test results:
| BTUs (Water Temp/Ambient Air/Relative Humidity) | |
| 80/80/80 | 138,000 |
| 80/80/63 | 129,000 |
| 80/50/63 | 88,000 |
It's efficiency drops when both air temp and air humidity go down.
An 17.9k gal pool will hold about 149k lbs of water and 1 BTU is 1F on 1lb of water.
In ideal conditions (80/80/80) that heat pump would be able to heat your pool about 0.9F per hour.
The same heat pump on (80/50/63 - air at 50F and RH at 63%) will only heat 0.6F per hour. If your relative humidity is lower it will heat even less per hour.
This is not an exact science as you have heat loss to the environment/evaporation and heat gain from sunlight (extra gain if using a solar cover), so actual heating is going to be different.
If your climate is indeed dry and receives a lot of sun then you might be better served with a Solar heater and if desired a gas heater as backup.
