If I understand correctly, the general rule of thumb on a TV screen is that if it's standard 480i/480p, you want your viewers to sit between 3 and six times as far the diagonal screen size in inches. The reason for this is picture clarity; at these distances, the image is viewable without any noticeable artifacts or quality issues.

So if I had a twenty-seven inch CRT, the ideal distance for viewing according to this formula is between 6.75 and 13.5 feet away. That's pretty close to our current viewing distance: 12 feet away (more or less, the old TV is quite thick).