What size do they mean by wide screen

[trt]

In measurements?

Is it taken from side to side or from corner to corner, like TVs?

Example: 20" wide-screen TFT LCD, 16:9 HD, 442.8x249 mm.

What is the screen size from left to right?

[vascop]

From corner to corner (the diagonal). Doing the maths I calculate that to be 442.8 mms wide as quoted in your post. If they are measuring the screen only then you would have to find out the width of the frame and add that on twice to get the physical width of the TV.

[VHG]

Wide screen is not so much about the "size" of the screen but the ratio between one "side" and the other.

Old fashioned TVs were sort of square so were called 4x3 (so not quite square but almost). So it was 4 units wide by 3 units high.

Widescreen TVs are often 16:9 ratio (not always and there are other ratios). So they are much wider than they are deep (bit like a letterbox).

The 20" is measured from corner to corner.