Multifamily p Tennessee Average apartment sizes fall to 10-year low Developers continue to add modern multifamily units to cities across the Midwest. And these new units? They're getting smaller. According to RENTCafe, the average size of apartment units completed in 2016 has fallen to 934 square feet. That's the smallest average size for new-construction apartment units since 2006, when the average size stood at 1,015 square feet. Overall, the average size of all U.S. apartment units is 889 square feet, according to RENTCafe.