I think that the thread you're looking for is
here.
Incidentally, if you are demo-ing and need to produce a passable head and shoulders portrait from life in a short time period, I reccommend the method that both Chris and I talk about on that thread: placing your hand on the canvas and using that as a rough guide, and make relative size measurements from that base, roughly 7".
I am going to teach this method this spring in my Phoenix class. I'm calling it the "Learned Hand" Method.