diff -u b/core/lib/Drupal/Core/Form/ConfirmFormHelper.php b/core/lib/Drupal/Core/Form/ConfirmFormHelper.php
--- b/core/lib/Drupal/Core/Form/ConfirmFormHelper.php
+++ b/core/lib/Drupal/Core/Form/ConfirmFormHelper.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Form;
 
+use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Url;
@@ -64,6 +65,9 @@
     $link['#title'] = $form->getCancelText();
     $link['#attributes'] = array(
       'class' => array('dialog-cancel'),
+      'data-dialog-options' => Json::encode(array(
+        'width' => 'auto',
+      )),
     );
 
     return $link;