Scrolling right-left is generally considered poor-usability. The default size works for the majority of computers… until such time that majority moves larger than 1024 pixel width. There are an increasing number of wide-screen monitors. This is one of the reasons wide-screen enthusiasts enjoy floating theme widths, specified in percentages rather than pixels.
With mobile technology advancing rapidly screen formats to accommodate these devices is the rule. Quite possible that in the not too distant future desktop and laptop computers will be overshadowed by mobile devices with graphic internet access.