I would base all seating on the concept that I will be doing my primary viewing while sitting in the comfy seats most of the time.
With that in mind, where do you like to sit in a theater?
Middle of theater is 1.5x the screen width. So, at about a 11' viewing distance, 100" really is 'center of theater', which is where that size comes from.
But that size is a recommendation, and nothing more. You can see how some people who are 14-16' away, may think 120" is ideal, when it may be closer to 133" for that distance. So, the THX recommendations are ideal to help people understand their viewing distance and screen size expectations.
But, if you like sitting a bit closer than 1.5x the screen width away, then go with a bigger screen. Some people sit 1:1 viewing distance to screen diagonal. I come pretty close sitting about 15' away from a 161" diagonal. I quite enjoy it, and my kids certain don't complain.
The one thing that is consistent, is that most well designed theaters have the middle seats aligned in such a way that when you are seated, your eyes are halfway up the screen.
That's the rule - 1/3 to 1/2 way up from the bottom of the screen is where your eyes should be. You do NOT want to be looking to far up at your screen if you can avoid it. Sometimes you can't avoid it of course.
