How to get the full size of attached image?

I currently have this in single.php:

<?php
      $attachments = get_children(array('post_parent' => get_the_ID(), 'post_type' => 'attachment', 'post_mime_type' => 'image', 'orderby' => 'menu_order'));
      if ( ! is_array($attachments) ) continue;
      $count = count($attachments);
      $first_attachment = array_shift($attachments);
      ?>
      <?php echo wp_get_attachment_image($first_attachment->ID); ?>

This displays the 150×150 thumbnail. What part this has to be changed in order to display the full size?