Display custom post type attached media file sizes

From your array, you should get the Media ID of the videos. If you have the media ID, it will be as listed below:

// Let's say you have the media ID
$meta = filesize( get_attached_file( $attachment_id ) );

By using the size format function you can list the size

echo size_format($meta);

Update according to your updated case scenario:

$custom_field_value="8785,8786,8787"; //If your custom field contains the IDs like this, simply replace my text with the custom field
if (!empty($custom_field_value)) {
    $media_ids = explode(',', $custom_field_value);
    if ($media_ids) {
        echo '<ul>';
        foreach ($media_ids as $media_id) {

            $file_size = size_format(filesize( get_attached_file( $media_id ) ));
            echo '<li>'.get_the_title($media_id).' - '.$file_size.'</li>';
        }
        echo '</ul>';
    }
}

Results: