Leo Tolstoy's grim conclusion is that we need about six feet. Now Leo's line of grim thinking led to the Soviet Union and we know how THAT turned out, so perhaps we can give ourselves a little more lee way and luxury.
The US census tracks the size of dwellings (ain't that a quaint word?) classified as single family homes. They've put up a readable summary of the data here but you can get the raw data in spreadsheet format on this page (it's this 'Median and Average Square Feet by Location' spreadsheet). Your tax dollars at work folks!
Our homes have been getting fatter with us! I doubt the number of people per family has been rising like this. (That fit is amazingly good, by the way. It's almost like the National Association of Single Family Home Builders has a chart for how large a home should be with year).
Does government legislate how much space a person needs? I could not find a federal number during my brief websearch (though I found mysterious references to the existence of guidelines, presumably passed on orally in some secret ceremony amongst government officials). On this page, we find that the city of Sacramento has put down 90 sq. ft of sleeping space for two tenants (the Fed allows two people per bedroom) and 50 sq. ft. per additional person.
Beyond this, the suitable size of a home is completely subjective. In my own experience, when I first saw the home we finally bought the rooms seemed gigantic (we were coming from a one bedroom apartment). On every subsequent visit over the next month (to take measurements etc.) the house seemed to get smaller (but cozier). When I first saw the house, my first reaction was that the house was too big for us. It gradually shrunk to be a smallish home, in my mind. Human beings get used to things so quickly.
In the end, like some other things in life, it's not the size of your house, but how you live in it (how you adjust and make the most of things).