Custom thumbnail size

Is there anyway to specify a custom thumbnail size. E.g.
wp_get_attachment_image($image_id, ‘thumbnail’);

Have the ability to specify a custom value in ‘thumbnail’. That way I could use a value from my functions.php

add_image_size(‘related_post_thumbnail’,212,150);

http://wordpress.org/extend/plugins/wordpress-23-related-posts-plugin/