Do not size from tub volume alone. A larger unit gives more recovery headroom in cold weather, while an undersized unit may run continuously and still heat slowly.
Water volume is the starting point
More water requires more energy for each degree of temperature rise. Use the actual spa volume rather than only the number of seats.
Climate changes output and COP
Heat pumps extract energy from outdoor air, so available output and efficiency change as outdoor temperature falls.
Steam & Oak publishes separate performance data for some models at warm and cooler air conditions, which is more useful than one headline COP.
Recovery time matters
A tub maintained continuously can use a smaller heat pump than a system expected to reheat quickly after being allowed to cool.
Cold-plunge systems that also use reversible heat pumps create a different sizing problem because cooling load matters too.
Hydraulic flow must match
Every heat pump has a recommended water-flow range. Too little flow can reduce heat transfer or cause flow faults; excessive flow can create unnecessary pressure drop.