Figuring out the right size generator you need in a power outage really boils down to two main questions:
(1) which appliances and devices you’ll run, and
(2) which of these you’ll run at the same time.
Before answering these, however, you’ll need to consider if the main purpose for this generator is emergency power at home or something else — for example, for portable power at a construction site.
With this in mind, to answer the first question think of all the devices you’ll want to operate in a short duration outage (1-2 days) and those you’ll need in a longer outage (longer than a couple of days).