Unless you live in a mansion, it is rare that you would ever need more than about 10KW and that's if you run everything in your house all at the same time. Most could be very comfortable with a 5KW to 7KW generator. Plus, the bigger the genset, the more fuel it burns, and the more often it needs refueling. If on gas, you're okay as long as the gas line isn't damaged.
I don't live in a mansion, but our house is 2500 sq ft, has a 3.5 ton central air conditioner, electric double oven, two refrigerators and a deep freezer, microwave, plus a bunch of TVs and all the normal stuff a house has. We have natural gas available (gas heat and hot water) and everyone we have gotten bids from have recommended a 17kw unit. The difference between a 14kw and a 20kw unit, once you factor in installation, is $1000 or less. If we are going to spend $8000 to $9000, I don't want to have think about turning something on.
We had a 4.5kw portable through Sandy...having to turn something off before we turned something else on got pretty tired awfully quickly. Then we couldn't get gasoline (none of the stations had power). Never again.
We have lived in this house for 19 years...in the first 15 years we lost power 3 or 4 times. In the last 3 years we have lost power over a half dozen times, and for more than a week 3 times in the past 2 years. In our view, having your own standby power is no longer a luxury. Electricity has gotten like indoor plumbing...it is a requirement for modern living.