I've got customers with single wide trailers with 20kw generators, and retired doctors with beach front homes running 6kw generators.
Then there was also the guy that didn't want to have to manage the loads, so he put in a 40kw generator so he could keep his pool house operational when the power was out as well.
There are just too many factors. Appliances being electric or propane is usually the biggest thing. After that is figuring out what size you need for the absolute necessities. Lights, water pump (if on a well), and enough heat to keep the pipes from freezing. Some way to cook food, which can be as simple as the microwave or a hot plate burner.
The basic loads can in almost every house I've seen, be handled by a 5kw generator, at most a 10kw.
Next comes the wants. Do you want to have a hot shower, use the electric dryer, or cook in the oven? Or do all three at the same time? Do you want to have the hot tub hot and operational? Or how about the steam shower (one guy with a 7kw generator was kind of unhappy that it wouldn't run his 9kw steam shower when the power was out).
Even with a 5kw most people can do one of the luxuries at a time, and the oven, hot water tank, and dryer are primarily resistive loads so the generator likes them better than big motors that have lots of start up surge.
How much of the load are you willing to manage yourself? Are you comfortable going to your breaker panel and shutting off the things you know you don't need? This is usually what determines the size needed. If you're willing to limit yourself and only use one appliance/major luxury at a time, then a 5 or 10KW generator is a great way to go. 1/2 to 1 gallon fuel burned per hour at full load. If you aren't willing to manage your load, and need to have a large enough generator to handle everything "just in case" all the appliances happen to get turned on or used at the same time, then you'd probably be better off with something other than a diesel, as diesels don't like being run without enough of a load on them for long periods of time. You'll be burning more fuel even if you aren't actually running anything, just to keep the engine/generator mass spinning.
One benefit of going with a non military generator and having it propane fueled , is propane never goes bad. You can buy a 1000 gallon tank or three of them, and if the power doesn't go out for the next 5 years no problems, as opposed to diesel, and especially gasoline, that you need to monitor the fuel's condition and either use it up before it goes bad, or make sure to keep it treated. Even if it's treated with stabilizers and biocides (algae and other stuff will grow in diesel, especially if there is any moisture in the tank at all) you'll still likely need a fuel company come out and polish the fuel every year or two to keep it usable (polishing usually involves treating it and running it through a series of filters to remove anything bad that might have gotten into it while sitting there)
