Hi Enter,
The 100% wide option will only work with a dynamic panel’s “Back Image” and “Back Color” options (shown in Anthony’s screenshot), and won’t affect the actual widgets within the panel itself.
If you want a rectangle widget to span the width of your prototype page, what you can do instead is use a “Set Size” action to set the width of the rectangle equal to the width of the browser window (which you can get using the “Window.width” function). Then if you place that Set Size action in the page’s “OnPageLoad” interaction, the rectangle widget will be resized to cover 100% of the page width when the page is loaded.