>>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.

I was going to make that point exactly. Here on the road I have my laptop, which has a screen that is fine for a laptop but nothing like 23". Since I am doing some stuff with .NET during off hours, and want all the screen real estate I can get for that, I added a 22" second monitor. The phenomenon you describe was quickly noticeable. I have kind of gotten used to it but still "hit the wall" sometimes.