Apple sponsored research claims that larger screen size improves productivity. As reported by Macword “the study, which evaluated Apple’s 30-inch Apple Cinema Display, concluded that large screens can offer gains of up to 50 percent to 65 percent in productivity on a variety of specific office tasks and can earn back their extra costs in time savings over several years.”
This is a great conclusion for Apple who is trying to sell their 30″ displays, but I would still recommend the use of dual monitors to increase productivity. The final screen size is relevant to the monitor resolution, which far too often is kept lower than optimal for a given screen. The use of dual monitors gives 2 primary benefits, doubling of total screen resolution and improved window management. With one large screen part of the productivity gains are lost during the time used to position windows within the large screen area, while dual monitors allows you to maximize windows within each monitor. For office workers having to multi-task between email, spreadsheets and documents I would recommend the use of a dual monitor set-up rather than a single large display.