custom-thumbnails.php

<?php

// add code below in your functions.php file in your active theme

add_theme_support( 'post-thumbnails' );

add_image_size( 'new-custom-featured-image', 800, 600, true );


add_filter( 'image_size_names_choose', 'custom_sizes' );
 
function custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'new-custom-featured-image' => __( 'New Custom Featured Image' ),
    ) );
}