Should I be considering 66 or 80 gallon heat pump water heater?

I would say that the tasks you’ve listed are far beyond what you’d be able to accomplish with a heat pump water heater. To simply meet the heating requirements of the house you’d need to have a dedicated boiler for the floor, and one that is sized to meet the heat load of a building that size. Here is out page that covers that topic, have a read -

All about heat pump water heaters

But speaking even just for your domestic hot water needs, I would suggest the bigger one. The thing about a hybrid heat pump water heater is that it’s in your best interest to keep it in heat pump mode, and only have it running off gas or electricity in rare cases of high demand for hot water. 

A larger reservoir would mean a greater chance that it can stay in heat pump mode, so to that end you would also be wise to install low-flow fixtures, and as that is a large house, I would also consider a hot water circulation loop which keeps the source of hot water closer to the fixtures. That will shorten the wait for hot water when you turn on a tap, and help it continue to operate as a heat pump as it will need to inject less cold water to replace what went down the drain while you waited. 

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