only in patch2:
unchanged:
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2042,3 +2042,27 @@ function system_update_8403() {
     }
   }
 }
+
+/**
+ * Cast 'size' setting of all entity_reference_autocomplete widgets to integers.
+ */
+function system_update_8404() {
+  $config_factory = \Drupal::configFactory();
+
+  foreach ($config_factory->listAll('core.entity_form_display') as $name) {
+    $config = $config_factory->getEditable($name);
+
+    foreach ($config->get('content') as $key => $component) {
+      $changed = FALSE;
+
+      if ($component['type'] == 'entity_reference_autcomplete' && isset($component['size']) && !is_integer($component['size'])) {
+        $component['size'] = (int) $component['size'];
+        $config->set("content.$key", $component);
+        $changed = TRUE;
+      }
+      if ($changed) {
+        $config->save();
+      }
+    }
+  }
+}