While it is about viewing angle, most people just don't think about that at all, and certainly won't calculate it. As well, while THX has come up with viewing angles for movie viewing, it isn't the same once you are in a family room, which may have much more social interaction where a TV should enhance, but not necessarily dominate, the environment.
In home theater, where the screen is 100% about creating the experience, I have found that most viewers have 10" to 12" of diagonal for each foot of viewing distance. This is across the board, with more people leaning towards larger, rather than smaller. So, for 10' of viewing, a 100" to 120" diagonal would be typical. But, that's cheap to create from front projection, and well out of the budget for a flat panel display and us mortal's budgets.
With flat panels, and normal family rooms, the size is almost always a 'bigger is better' reality with the most common complaint of TV purchasers being "I wish I had bought the next size up".
To that point, I had a client with a 70" TV that they had just purchased for their basement theater setup, and they were wondering what to put in the family room. I told them I was going to take their 70" TV and put it in their family room and take a 110" projection screen and put it in their basement. I told them to trust me and that they would thank me for that decision within a week. I was wrong. Before a couple days had passed they had already thanked me.
At 10' away, there is almost no situation where a 70"-80" TV isn't appropriate, acceptable, and good looking to almost all people. Once the shock of the size wears off, which only takes a few days, you start realizing that even that big 65" TV you bought, that you are sitting 12' away from, really isn't all that large, and maybe the 75" or 85" TV would have been the way to go.
Yes, it would have been.
* - 85" TV in my family room with couch at 12' and kitchen/eat-in area behind it. 161" front projection in basement.
