W2R
Moderator Emeritus
A quick and drty web search reveals that house sizes averaged 1400sf in 1970, and increased to 2500sf by 2006.
Interesting! So, I pursued it. Your figures are about right. The Census says here that the average house size in 1970 was 1500 square feet, and the average house size in 2007 was 2277 square feet. The median house size in 1970 was 1385 square feet, and in 2007 was 2521 square feet.