Pretty sure it's wherever they want it to be. I worked in a hospital that had huge ward icus and they would literally put "isolation" tape in a six foot square around a bed.
We staff knew it was as ridiculous as it looked but went along with it.
In the novel "one flew over the cuckoo's nest" one of the characters keeps getting touched by another, feeling threatened. He cried over and over "just touch me again! just touch me again!", the line being redrawn with every touch.