How do I display the attachment's file size of a node?
You can do this hardcoded with the $node object by doing so (preferably in theme_preprocess_node in template.php) :

If you field field_my_field as a single value :

$file = field_get_items('node', $node, 'field_my_file');
if ($file) {
    $file_size = $file[0]['filesize'];
    $file_size_formated = format_size($file_size);
}


If you field field_my_field as a multiple value :

$files = field_get_items('node', $node, 'field_my_file');
if ($files) {
    $files_size_formated = array();
    foreach ($files as $key =&gt; $file) {
        $file_size = $file['filesize'];
        $files_size_formated[] = format_size($file_size);
    }
}


In template.php you should have something like this :

// This function allow one preprocess_node function per content type
function mytheme_preprocess_node(&amp;$vars) {
  $node = &amp;$vars['node'];
  // Preprocess function for each node type
  $function = __FUNCTION__ . '_' . $node-&gt;type;
  if (function_exists($function)) {
    $function($vars);
  }
}

function mytheme_preprocess_node_my_content_type(&amp;$vars) {
  $node = &amp;$vars['node'];
  $file = field_get_items('node', $node, 'field_my_file');
  if ($file) {
      $file_size = $file[0]['filesize'];
      $vars['file_size_formated'] = format_size($file_size);
  }
}


Then in you node--my-content-type.tpl.php you can print your var $file_size_formated where you want like this for example :

&lt;div class="file-size"&gt;&lt;?php print $file_size_formated; ?&gt;&lt;/div&gt;