Well, screensize is not a good measure for this kind of things. It should be the viewport size. Because in my Windows 7 I have the task bar fixed to the left of the screen. So this might be my problem.
But I think I am not the only one doing this, especially on widescreen monitors with a smaller resolution (e.g. notebook), this should be very common.
On a bigger screen I would probably have two windows open. For example when passing a questionnaire in word format to Limesurvey, then I would run half of the screen (minus the left bar) for the browser with limesurvey and half the screen for word. I think this is a common workflow with big screens.
Is there a way to make it the viewport width - 20px instead of the screen width - 20px? I think this would increase usability.