Generac publishes usage tables against loads, and that is the easiest way to sort out how much propane you would need.
@PJRAM brings up a good point that full load is not a sustained full load unless you get a Prime power rated generator. (Basically one designed to be on at 100% power all the time, in high heat.)
@grsthegreat is the expert, but I would try to target a generator that can run happily at about 50% when your normal loads are on, but pay extra attention to your AC units and well pump to ensure that your generator has enough capacity to start them up. LRA (startup amperage, aka "locked rotor amperage") can be 3-7 times running power, and your generator will need enough capacity to be able to start the AC. Inverter AC units, aka variable speed (infinitely variable, not three speed) ACs typically have startup currents very close to running currents. My $0.02 is to upgrade your AC when you are installing whole house standby backup power. It makes the load management much easier, and makes the demands on the generator much more reasonable.
I think that you should pretty much count on not getting refilled during an emergency. Firms almost always prioritize getting propane for heating out first. Our local branch of a national propane company told me when I asked that when there is an emergency, they don't fill generator propane tanks as a matter of policy until the the emergency is over and they are caught up with heating fuel deliveries.
You also need to look at the evaporation tables that will de-rate the flow from a propane tank by temperature and load to make sure that when it is cold you can get enough fuel out of the tank. That is mostly a small tank, above ground tank issue. Personally, I would take the longest outage that I had ever seen and double it. (at least)
All the best,
Peter