>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?

I have the same setup as Charles, a 27" + a 19" for the same reasons you have, my desk does not have more space, and I do not have any troubles at all, the only problem is that when I play a game on the 27" and then move to the 19" (usually I am reading or listening to music while playing) then the game acts weird for it thinks I am trying to move hard left as it does not realize I left it's window, but that is not related to the size of the monitors at all.