We originally sized at 20 kW for 3,400 sq foot house. Then Briggs discontinued the 20 kW and upped it to 26 kW for the same price. We’ve got LOTS of power! We didn’t want the oven going out when we are cooking Christmas dinner.
The electrician did all the sizing calcs (which aren’t hard to do). But you need to be aware of high inrush current when devices start which complicates the design a bit. If you lost power with a lot of house loads on (clothes dryer, AC, oven), you won’t just be able to throw the generator on. You have to disconnect some of those loads and then sequence them back on. The “load shed” modules we bought do that automatically for us.
My system is a bit older school...
Manual, with separate circuits for the critical items. Fortunately I’ve never had to stress test it.