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
.