The table below summarizes these conversions. Select the TV size from the top (in inches) or bottom (in centimeter), select the viewing distance from the left (in feet) or the right (in meters) and you get the equivalent diagonal field of view.
|Conversion from TV diagonal size to HMD field of view|
Field of View = 2 * arctan ( (S/2) / D )
Just make sure that S and D are using the same units and remember that most arctan functions return the value in radians, not degrees.
From the table, you can see that a standard-issue professional HMD with 60 degree field of view is equivalent to about a 70" screen from 5 feet away whereas a wide field of view device such as the xSight 6123 with 123 degrees field of view is like a 90" screen from 2 feet away.
How much field of view is enough? This is truly application-dependent. Most people prefer watching TV on a 50 inch screen instead of a 24 inch screen, but would prefer using a 24 inch computer monitor for word processing to a 50 inch monitor. Wide field of view - and thus immersion - is great for gaming. High pixel density - and thus narrowed field of view with a given resolution - is better for seeing details.
To illustrate the difference between TV requirements and goggle requirements, consider this: for TVs, THX recommends “best seat-to-screen distance” is one where the view angle approximates 40 degrees. 40 degrees in goggles is quite small.
UPDATE: Please also see the matching post on "converting diagonal field of view and aspect ratio to horizontal and vertical field of view"