>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?

Running different size monitors is not a problem.

For what it's worth though, 3 monitors is the sweetspot. I run 3x24" @ 1920x1200 each = acres of screen real estate.

I would buy another desk to make it happen.