How big a generator do I need.?
Don't go for the smallest you can get away with. A small unit straining can use as much fuel as a better unit loafing and you may need more power for tools, battery chargers, hair curlers, etc. Women will not be denied their comforts. Are we tallkng a portable unit? With a heatpump I would like a 10-12 kw minimum and a 20 kw with an automatic switch with tied to the main panel for two heatpumps and complete comfort. Since the heatpumps need more power to start than when running I like to add a delay timer to one of the air units so they don't start up at the same time when the gen-set kicks on. An often overlooked consideration is fuel source. Think about your area and what might become scarce or troublesome to get like fuel station lines and shortages, wasting one fuel to go get another and poor weather or travel conditions. In my area, no matter how bad the disaster, natural gas to the house was always working so that's what I recommend. Propane is another option for longer run times and alternative cooking fuel but that entails an on site storage tank. Propane stores well and, along with natural gas, won't gum up the lines or require stabilizing like gas or diesel fuel. Good luck
The whole key to this answer is fuel. If you live where you can get lp or natural gas to the house then go with the generac whole house with automatic switch. I mean a constant supply like city gas or propane. NOT FROM A TRUCK. The trucks will not run if your in the middle of a hurricane or other types of storms If you have to use propane from a tank, rent the largest one available. A 300 pound tank will only last 4 days under full load. Gasoline is dangerous and to hard to keep fresh.
Heat pumps are pretty big users of power. I would get at least a 3500 watt but that would be good for heat, fridge, a couple lights and a tv only. Bigger would be better. It may consume 5-10 gallons of gasoline per day. Running your own generator is not cheap.