Multiple Post Thumbnails image size

hi!
on http://www.virtual-reality-games.org
i added a secondary image to each post, shows up, everything good
function.php:

if (class_exists('MultiPostThumbnails')) {
    new MultiPostThumbnails(
        array(
            'label' => 'Secondary Image',
            'id' => 'secondary-image',
            'post_type' => 'post'
        )
    );
}

content.php:

<?php
if (class_exists('MultiPostThumbnails')) :
MultiPostThumbnails::the_post_thumbnail(get_post_type(), 'secondary-image');
endif;
?>

only the size is not good
it displays as thumbnail size but i want it to display 550×350

tried to add image size in function.php, doesn’t work

add_image_size( 'custom-size', 220, 180, true );

tried to define size in content.php, doesn’t work

<?php
if (class_exists('MultiPostThumbnails')) :
MultiPostThumbnails::the_post_thumbnail(get_post_type(), 'secondary-image', set_post_thumbnail_size( 550, 350, true ));
endif;
?>

how can the secondary image be displayed 550×350?
what am i doing wrong?

thanks in advance