I put a 1000 gal tank in for my backup generator. Remember they only fill to 70% to allow for expansion. So 700 gallons. If your tank is supplying other heaters or devices consider your consumption.
if you are purchasing a tank you might want to consider what the cost is for a larger tank vs multiple smaller tanks. Especially if you can sell your smaller tanks on location.
I bought my house/property with a 250 gal in place and sold it back to the propane company for a $550 credit. YMMV.
Maybe it varies by location, but in my area tanks can be filled to 80%, so my 250 gallon tank can hold 200 gallons. However my 24Kw generator when running at full load will use nearly 4 gph. Our home is all electric. Normally, we would be running closer to half load (or about 2.5 gph) , but there are situations in the winter time where full load is required.
In fact, I have a Load Shedding Switch to disable 1/2 of the heat strips because of the huge draw they have on the unit. I can override that if need be and manually 'manage' what I want to run at the breaker panel, but my unit is set up to run Whole House if needed.
All that said to point out that fuel consumption can be quite high. So each person needs to consider for what purpose you use/need your generator. IF only for convenience sake, for short term outages then a smaller tank might be OK. But IF you have elderly, infirmed, run a business from your home or experience long term outages (days) then you need to have a sufficiently large tank.
In my situation (best case scenario....1/2 load) I could run my genset about 3 days before needing to refill the tank (with a 250 gallon tank). So do your homework folks. You don't want have to re-do anything. ONE installation done to meet your worst expected needs is the way to go (finances permitting).