Okay,
I just found the answer myself. ??
You can change the header image size in the functions.php line 22-27
// Custom header
$args = array(
'width' => 1280,
'height' => 416,
'default-image' => get_template_directory_uri() . '/images/header.jpg',
'uploads' => true,
'header-text' => false
);
just change the width and height but try to keep the aspect ratio.