The Simple Template restricts a blogs header by placing it inside of the <div class='content-inner'> container. This box is the one controlled by the Template Designer sliders and since your header is inside that box it has the same width. Overcoming this restriction is really straight forward, simply move the header code outside of this restricted area.
Editing HTML is the Key
Start by opening up your blog in Design->Edit HTML mode and search, ctrl+f, for <header>. Select all of the HTML from this point until you see a tag like this, </header>. Now you must cut this text, ctrl+x, so it may be moved to another container. That's the hard part, make sure you get the entire header code or else things will likely go wrong.
Now for the easy part search for <div class='content'>, type this in the search box otherwise the cut code will be lost and you'll need to start all over. If this isn't found, try looking for double quotes <div class="content">. Once found simply paste the header code with ctrl+v below this line and preview your changes. If you've done everything right the header should now span the full width of your display. Now, keep in mind this might not look exactly how you want it and further tweaking may be required. Once satisfied save your changes or if it's just not to your liking, discard them.
Altering the Blogger Simple template to allow a full width header is pretty easy. All you need to do is move the header code from one place to another. This works best with large graphic headers and getting the exact look you want may take some work. In future discussions I may cover more advanced customization of full width headers, if there is interest just ask in the comments.