Hi, in sf-header.php find the code
$header_output .= '<header id="header" class="clearfix">'. "\n";
$header_output .= '<div class="container">'. "\n";
$header_output .= '<div class="row">'. "\n";
$header_output .= sf_logo('span4 logo-left');
$header_output .= '<div class="header-right span8">'.sf_aux_links('header-menu').'</div>'. "\n";
$header_output .= '</div> <!-- CLOSE .row -->'. "\n";
$header_output .= '</div> <!-- CLOSE .container -->'. "\n";
$header_output .= '</header>'. "\n";
$header_output .= sf_mobile_search();
$header_output .= '<div id="main-nav">'. "\n";
$header_output .= sf_main_menu('main-navigation', 'full');
$header_output .= '</div>'. "\n";
and change the span according to your liking!