A standard CRT uses a 3-4-5 triangle. An LCD is a little taller (a 4-5-x triangle) so you need to use Pythagorean theorem which is:
4^2 + 5^2 = DIAM^2
A 17" LCD is about 141 sq inches
That comes to about a 10.63" high by 13.29" wide screen
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A 19" LCD is 176 sq inches with a viewport measuring 11.88" high by 14.84 wide