The Design Area is closely relates to the Settings Panel.  In the Design Area, there are many items which can be adjusted to fit the feel of your site.

 

To adjust the dimensions of the border, you can use your mouse to drag and drop the edges, or you can enter a new number for Height and Width under Background.  You can also change the location of the Search Box and the Search Button by dragging them with your mouse.  When you select one of those objects, you will see the lower section of the Settings Panel change and display the settings for the other object.  For more information on the settings for each object, see the Settings Panel area of this help file.