Here's a quick guide for any of you looking to get a new TV. There are 2 competing standards which specify the ideal viewing angle for a good cinematic experience.
THX states that the ideal minimum viewing angle is 36 degrees to the farthest seat, with 26 degrees being a minimum.
SMPTE standard EG-18-1994 recommends a minimum angle of 30 degrees.
Here's a good calculator for us to use:
Let's say my room has a 12' viewing distance. It's a pretty standard sized room, not too big.
From this, we see that to meet the minimum SMPTE standard, we need a 75.6" wide screen, which means a 94.5" diagonal for a 4:3 TV screen, or an 86.7" diagonal for a widescreen 16:9 TV.
To meet the ideal minimum 36 degrees for THX, we need a 90.8" wide screen, which translates to a 113.5" diagonal 4:3 TV Screen, or a 104.2" diagonal 16:9 screen.
Let's look at from the standpoint of a TV.
If you have a 30" widescreen (16:9) TV, you should sit no more than 3.5 feet from it for the recommended 36 degree viewing angle.
If you have a 34" widescreen (16:9) TV, you should sit no more than 3.9 feet from it for the recommended 36 degree viewing angle, with 5.4 feet being an absolute max at 26 degrees.
If you have a 36" standard 4:3 TV, you should sit no more than 3.8 feet from it for the recommended 36 degrees, with 5.3 feet being an absolute max at 26 degrees.
For a widescreen 53" projection TV (16:9), you should sit no more than 6.1 feet from it for 36 degree angle, and no more than 8.5 feet as an absolute maximum for a 26 degree viewing angle.
One thing to note is these guidelines are simply a baseline for a good cinematic viewing experience. A wider viewing angle will give you a more immersive experience and lend to the "suspension of disbelief" that we want to accomplish when watching a video. Larger angles will also reduce eyestrain (though this can also be accomplished with D65 bias lighting).