>>>If you want to use your PC with two monitors, how important is to have both of the same size? The reason I am asking is I have now one monitor (23") and want to add another one. But I don't have much room on my desk for another 23" so I am thinking of getting a 19" monitor as the second one. Will it be a problem?
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>>The only problem you will have, is when you move the mouse from the bigger monitor to the smaller one. If the mouse pointer is above (or below, depending on the setup) the max height of the other one, your mouse has nowhere to move, and will refuse to move horizontally. The fix is of course to move the mouse vertically first. It will feel odd the first couple of times.
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>But if you adjust the relative position of the monitors to reflect the real-world positions in set up thenthe reason for the problem is pretty intuitive?

With two dedicated monitors, yes. I have a laptop on my desk and a separate monitor, so it's not possible to align them vertically.