I've seen tiny houses in architectural magazines of about 100-150 sq. ft. I could live in that for awhile, but there would have to be a compelling reason and a great outdoor climate to do it for more than a few months!
On the other hand, I had a friend who built her own tiny house on property she owned in Oregon. It was about 175 sq. ft., no electricity, and no sanitary, just fresh cold water. She lived there in the summers for several years.