Did the calculations here w/ several buildings, heat pumps, window units in barn-loft, pool, well, washer/dryer, etc, not to mention heat strips in HVAC if it's cold, and wound up with better than 40kw at start load. Sure, EVERYTHING will never start at the same time but I wanted to be able to do what I wanted post-hurricane or ice-storm. Opal really sucked with only a 3500 watt generator for a week, nothing like running a chainsaw all day and then sweating all night under a box-fan while the Briggs & Stratton genset howled and churned outside! So I've got a 40kw set, the only cost for excess here is fuel, it's not a full time thing, just emergencies so what the hey... Figure what you want/have to run and do your load calculations.
Also, three phase is a problem for residential use. Yes, you can pull 120v off each leg but the sine waves are off for 240v, those appliances will not last long in that environment! Look around, there ARE bigger single phase gensets out there, usually '12 wire' that are configurable to different outputs...
I used a Cutler Hamner (sp?) 400 amp manual switch as this rig is for extended outages, don't need auto switching as the power is out regularly around here for 5-10 minutes at a time, got it on Ebay for $100 or so in almost new condition... The switch is mandatory, don't play with linemen's lives wiring a generator into your panel and just flipping your main breaker to 'off'!!