Bear in mind. If you make your logo stretch across the entire top of the page, your users may have to scroll left and or right to see the entire site if they are running a lower resolution on their Monitor. Or if they have a smaller monitor.

The first thing you need to do in order for the logo to go across the whole top is to modify the
header.html file in the theme.

As it is now, the top table on that theme where the logo resides is split into two columns. A left column(60% width) for the logo and a right column(40% width) for your banner/ads(if you use them).
You would need to change that table to only have one column and then move your banner code to display your banner/ads(if you use them) above the entire page.