I don't know if this is the right way but I have found that if you change the header width the rest of the site will follow
open your overall_header.tpl cut all the code below {QBARS} to <END>
Then add the table below change the width to what you want. 100 = 100 pixels and then paste what you cut from overall_header.tpl to the space labeled "paste here" in the table below.
<table> <tr> <td>Paste Here</td> </tr> </table>
Like I said I don't know if this is the right way!!! This has worked form me but I usually make my own themes.