Managed to find how to do it. If anyone is interested in what I did:
in functions.php, change the size (width, height values) in 2 place:
1.---------------------------------------------
<?php
// No CSS, just IMG call
define('HEADER_TEXTCOLOR', '006163');
define('HEADER_IMAGE', '%s/images/spring_flavour/header_bg.jpg'); // %s is theme dir uri
define('HEADER_IMAGE_WIDTH', 704); <----Here
define('HEADER_IMAGE_HEIGHT', 200);<----Here
function nigarila_admin_header_style() {
?>
2.---------------------------------------------
<?php
}
function nigarila_header_style() {
?>
<style type="text/css">
#header {
background: url(<?php header_image(); ?>) no-repeat;
height:200px; <----Here
padding:0 0 0 18px;
}
#header h1 a
{
color:#<?php header_textcolor();?>;
}
</style>
<?php
Good luck