Transverse diagonal measurement has been proposed as a predictive method.
A good rule of thumb is that your distance from the screen should be 1 times its diagonal measurement.
The diagonal measurement of a display as the actual viewable area is also dependent on the aspect ratio of the display.
Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangle was equivalent to the diameter of the tube's face.
When monitors are sold, the quoted size is the diagonal measurement of the display area.
(Remember, these are diagonal measurements, so that's a huge difference.)
Unlike most VCR's, videodisks yield a sharp and clearly detailed picture, even on screens with diagonal measurements of 40 inches or more.
When displays are marketed the quoted size usually is the diagonal measurement of the display area.
The 40" class has a diagonal measurement of 38.5".
In doing this, they limit the diameter of the image projected to slightly more than the diagonal measurement of the photosensor.