The nation's love affair with McMansions continues to wane: The average size of new single-family homes completed last year fell again, a trend expected to continue for several years, the National Association of Home Builders said at its annual conference in Orlando.

In 2010, completed homes measured an average 2,377 square feet, down about 3% from the year earlier. By 2015, many builders expect the average home size to shrink to 2,152 square feet.

This...