There are sizing charts online. It really depends on the appliances you have and what you want to run. Electric heat, hot water heater, dryer, stove, Hot tub all add up fast.
The automatic transfer switch is pretty slick. We lose power and it takes about 10 seconds before the Generac kicks on and takes over. When power returns, I never know. The ATS just flips back to Electric co. no interruption. btw- with Tesla powerwall you don't even lose those 10 seconds of power. But it will cost a crapload.
Once this year we had a huge outage in my area that included cell towers and cable service. We had power but no TV/nternet or cell service. That was unusual. Normally those are unaffected.