All the above is absolutely correct, there must be absolutely no way your generator's output can ever be backfed into the power grid. But on to the size question. I installed a propane-fired genset last year for our house, which is about the same size as yours. I chose 10KW, which is not quite large enough for the full-house load. But I only dropped a couple of 120V circuits, the clothes dryer, and my shop circuits. Everything else, well pump, reefer, hot water heater (outages mean work on the farm, which means getting even dirtier than usual, and I don't like cold showers!) is covered on the automatic transfer switch. I am really pleased with the setup. But for a little better than 10 years I had been using a 4KW portable to run the reefer, furnace, well pump, and a few 120V light circuits. I did not use a transfer switch for that. Instead, I converted the hard-wired pump and furnace, both 120V, to plug and receptacle, so everything was run off extension cords. A big nuisance and very messy, but totally safe (I think others here have done the same). This kept lights and heat on, the reefer cold, even the tv and computer could be used (that genset has much more stable output than I ever expected). I hope I'm not being too long-winded, but the change-over to a permanent installation was occasioned by the big winter storms of the winter before last. I decided that at my age - pushing 70 - it was time to make life a little easier. It's turned out to be a great investment, I loved it when the first time the lights went out and my wife turned to me and said, "now what?" and my reply was, "just count to 15...". Good luck with your project, hope this has been a little helpful.
