Need to help on the size of "Featured Image" for Graphy theme?

Hi,

I just installed this theme today, and was playing around quite happily, until found out that “Featured Image” is shown only in 640 wide and 198 high on my article, despite my Feature Image setting was 768 wide and 1024 high. Regardless of the original image size, the end result was the same; I got 640 wide and 198 high only.

When I checked Function.php file, this is what I see.

========================
/*
* Enable support for Post Thumbnails on posts and pages.
*
* @link https://codex.www.ads-software.com/Function_Reference/add_theme_support#Post_Thumbnails
*/
add_theme_support( ‘post-thumbnails’ );
set_post_thumbnail_size( 800 );
add_image_size( ‘graphy-page-thumbnail’, 1260, 350, true );

// This theme uses wp_nav_menu() in one location.
register_nav_menus( array(
‘primary’ => __( ‘Primary Menu’, ‘graphy’ ),
) );

// Enable support for Post Formats.
add_theme_support( ‘post-formats’, array( ‘aside’, ‘audio’, ‘chat’, ‘gallery’, ‘image’, ‘link’, ‘quote’, ‘status’, ‘video’ ) );

// Setup the WordPress core custom header feature.
add_theme_support( ‘custom-header’, apply_filters( ‘graphy_custom_header_args’, array(
‘default-image’ => ”,
‘width’ => 1260,
‘height’ => 350,
‘flex-height’ => false,
‘header-text’ => false,
) ) );
========================

I have tried changing the height above to 768 / 1260, but end results were the same. The Featured Image works, but shown only in 640 wide and 198 high. I have deactivated plugins, but to no avail.

I really need help why my Featured Images are not shown in the size I have selected when I selected them from Media Library.

Any advice and suggestion will be appreciated.
Thanks!