What I find interesting is that right at the start of the THX link...
http://www.thx.com/home/setup/display.html
They do show that seating from the TV should be at 40 degrees or less viewing angle and recommend 6-9 feet from a 60" diagonal. 36 degrees is at 6.7' from that size screen.
This also means, that the 6-9 feet represents a range from 40 degrees to 26 degrees (or very close).
In simplest terms, 26 degrees is the furthest that THX wants you to view a screen from and 40 degrees is the closest. 36 degrees is towards the front of that range, and is where I tend to stick and likely will stick for a number of years.
Since most people tend to not always watch the greatest content, and want versatility in their setup and already tend to think that a 60" display is 'HUGE!' - it makes sense to try to talk them into 1.5x width as their viewing distance. Or, at least the distance to their primary seating location - typically the middle seats.
Then, you have to balance it
all against personal preference. THX is a guideline, and it's one that many theaters follow. So, it's the personal preferences of individuals working within this guideline that truly determines screen size.
Some people also look at their environment which is often less than ideal. If your room is a bowling alley, then the screen either fills an entire wall and looks lousy, or you make it smaller and must sit closer - yet the final result fits the room far better.
There's a lot of science to it all, but the real art of it is the allowance of personal preferences to be the true dictator of size.
