>>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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>Hrm... I wonder if it's possible to have one monitor in landscape orientation and another in portrait orientation...

Depends upon your video card(s) - but generally yes.