Images in the Twenty Fourteen theme

Well, that was… interesting. In a nutshell there seem to be two things in play here:

• max_posts set to 6 in add_theme_support( 'featured-content'
• quantity set to default 6 in Featured_Content class

quantity defines how many posts there are, but can be no larger than max_posts. However while result can be pushed down by changing max_posts it cannot be pushed up since there doesn’t seem to be clean way provided for changing quantity. If I read code accurately it was either meant to be input in Customizer or was there and got removed.

So to change both, this is closest to sane I could come up with on Sunday evening:

add_action(
        'after_setup_theme',
        function () {
            add_theme_support(
                    'featured-content',
                    array(
                            'featured_content_filter' => 'twentyfourteen_get_featured_posts',
                            'max_posts'               => 8,
                    )
            );
        },
        11
);

add_filter(
        'option_featured-content',
        function ( $array ) {
            if ( ! empty( $array ) && is_array( $array ) ) {
                $array['quantity'] = 8;
            }

            return $array;
        }
);

Note that posts set is cached in transient, so need to re-save it with Customizer for new count to apply.

As for size – thumbnail size is used for grid version of it, see content-featured-post.php.