The best answer is really "What you prefer".
Most people find an ideal spot by measuring the panel corner to corner (in this case, 40 inches), and then sitting back 2-3x that. 7 feet would be 84 inches, and 10 feet would be 120 inches.
If you watch a lot of TV, find a panel size that you can view at a distance and not move your eyes to see the whole screen, as that causes eye fatigue.
That said, some people love to buy a big screen and sit close. We're okay with that, too! :)
--Mr. S