Look in smart_template/custom/themes
Just edit your default theme css file.
Look for the Logo entries - Ours are full width and 100px high which sit on top of the header background with is 180px high and includes the background for the speedbar.
Code:
/*--------------------------------------------------------
LOGO
--------------------------------------------------------*/
/* Adjust Logo Width & Height Dimensions Here : Default W480 x H100 */
#st_logo h1 { background: url(sky/logo.png) no-repeat; width: 900px; height: 100px; border: 0; overflow: hidden; margin: 0; padding: 0; z-index: 30; float: left; }
#st_logo h1 a { display: block; text-indent: -10000px; width: 900px; height: 100px; border: 0; text-decoration: none; }
#st_logo h1 a:hover { border: 0; text-decoration: none; }