If a screen is said to be “19 inches” that means that the viewable area is approximately 19 inches along the diagonal, i.e. corner to corner. Even that doesn’t tell you much, but I have found an excellent table that gives the true dimensions of many screen sizes.
It also gives the pixel size, which is a good measure of the readability of a screen. Anything less than 0.297 mm may make everything too small unless you alter the screen’s resolution. Doing that is fine but it will reduce the sharpness of the image.
Everyone’s vision is different so there’s no substitute for seeing a screen in action before buying it. Failing that, look up the pixel size of the screen you’re using now and compare it with the one that you’re considering buying.
NB: your current screen resolution may be different to that which is listed in the table. E.g., the table says that a 15″ screen at a resolution of 1024×768 has a pixel size of 0.2977 mm. If you have a 15″ screen set at 800×600 to make it readable then a pixel size of 0.2977 mm is too small for you.
To find your current screen resolution in Windows XP click Start>Settings>Control Panel>Display>Settings. To exit without changing anything click Cancel.
This topic is quite difficult to explain so hopefully that all makes sense 
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