Well my average day in and day out rate is around 2 KW's an hour but as my utilities usage tracker shows my low points are under 300 watts an hour but my peaks can be over 20 KW's an hour.
Figure your monthly utility KWH usage divided by the number of hours in your billing cycle and then multiply that average number by 10 and that would likely be the upper end of what size of generator you would need for normal day in and day out operation.
With proper peak demand control I wold say that you might be able to get by with a genset that has as little as 2 - 3 times your average power consumption.
I know that if I absolutely have to I can run on as little as a 3000 watt genset but for reasonable emergency living without having to shut down my electric water heater, electric dryer, stove and well pump I can run just fine off my old 10 K unit. I just cant run my high load equipment in my shop though.
Given that if you have a fairly low average power usage and can isolate your high load equipment and have that only run at specific times you could run a dual gen set system with a much smaller unit for the continuous load applications and have second much larger unit you only start when needed for the times when you will be running high demand loads.