That's not going to work, instead just use `wp_get_attachment_image_src` , it takes a size parameter.

    // get the $attachment_id
    $attach_stuff = wp_get_attachment_image_src( $attachment_id, 'full');
    var_dump($attach_stuff);


You will be returned an array fo values to work with:

    [0] => url
    [1] => width
    [2] => height
    [3] => boolean: true if $url is a resized image, false if it is the original.

I am using `var_dump` in my example because you need to turn on [**debugging.**][1]


  [1]: http://codex.wordpress.org/Debugging_in_WordPress