Ezra Klein has a post responding to a suggestion of mine--discussing the difference in housing size among the nations and cities. The US is sitting at something over 800 square feet per person (that's including all square footage in the house); European figures, which are "usable space", presumably just kitchens, living rooms, bed rooms, range from 300 for Estonia to about 550 for Denmark
US median housing size has gone from about 1500 in 1973 to 2500 in 2005, with the Northeast leading the way.
I doubt how much value the increased square footage adds to human happiness. After all, it only takes 100-200 square feet to give each child a bedroom.