You have to measure the diagonal of your display by measuring from one corner to the opposite one.
You might have to convert from cm to inches. 1 inch is 2,54 cm, so a 19" display would also be a 48cm display.
When manufacturing a Display there can be errors in some pixels. This will result in the pixel or sub-pixel not letting any light pass or never blocking light. If a complete pixel is defective it will either be black or white. However, the same error can also occur in a subpixel, which will then result in it always/never displaying its respective color (either red, green or blue). Due to the very high number of pixels such errors are almost impossible to avoid, and that is why displays with a few pixel errors are still shipped.
Vertical or horizontal lines are almost always a sign for a display damage.
A display damage is also indicated by lines that (dis-)appear when the display is pressed on.
In this case the display has to be exchanged.
If the Display is black, but the Desktop can still be seen a bit, this means, that the background light has died.
Some of the errors could be:
In most cases it is probably best if you send your device in for repair to a specialist.