You can change the width of the Design in the Design Gallery but
understand the Header does not stretch with it since it would distort
it. The standard parameters for the 4.0 Design are 750 pixels wide. You can
change this for a wider alternative by going into the 4.0 Design Gallery
and make an adjustment to the "page settings" area. You will see the
page width is set at 750 but you can play with the numbers a little and
go up to 850 or 900 or even choose "justify" which will put it at 100%.
Then hit enter or scroll down to the bottom of the page and click
"save". View your website and see if this is how wide you wanted to make
the Design. Keep in mind not everyone's monitors are the same size. View your website and see if you like the new look!