Design Area

Internet Explorer 8.0.6001.18702
800x600 pixels

A design area set to 800x600 (by JavaScript) includes the entire Internet Explorer window with the following selected:

Menu Bar (File, Edit, View...)
Compatibility View Button
Command Bar (icons & words right of Favorites & tabs)
Status Bar (at the bottom, usually says "Done")
Lock the Toolbars

It also includes the title bar, address bar and the side margins, so you actually get only 720x432 pixels to design in without scrolling.