Your header image width changes according to your container width settings in Graphene Options –> Display –> Column Width Options. Option to change the header height will be available in Graphene version 1.7
Or here is the custom header function to manipulate slider dimension,
function graphene_custom_header_width(){
return 1000;
}
function graphene_custom_header_height(){
return 250;
}
add_filter('graphene_header_image_width', 'graphene_custom_header_width');
add_filter('graphene_header_image_height', 'graphene_custom_header_height');
And custom CSS,
#header {
height: 250px;
width: 1000px;
}
