You need to add this to the child theme’s functions.php
function graphene_custom_header_width(){
return 960;
}
function graphene_custom_header_height(){
return 157;
}
add_filter('graphene_header_image_width', 'graphene_custom_header_width');
add_filter('graphene_header_image_height', 'graphene_custom_header_height');
And this to your child theme’s stylesheet or to the Custom CSS,
#header {
height: 157px;
width: 960px;
}