Using taxonomy to give option of page layout - working but showing error (Notice: Undefined variable: node in include())
The page.tpl.php template file will receive a $node variable only if the user is visiting a node page; in other cases, it will not get any $node variable, even if the template file is still used (if a module doesn't suggest another template file that takes the precedence).
In fact, the documentation for page.tpl.php clearly describes the $node variable as follows. (Emphasis is mine.)


  $node: The node object, if there is an automatically-loaded node associated with the page, and the node ID is the second argument in the page's path (e.g. node/12345 and node/12345/revisions, but not comment/reply/12345).


Before accessing $node, you should be sure it has been set.

if (isset($node)) {
  $field_page_width = field_view_field('node', $node, 'field_page_width');
}


Also, field_view_field() is used to render a field. If you just need to get its value, you should use field_get_items().

if (isset($node)) {
  $items = field_get_items('node', $node, 'field_page_width');
}