MAGETWO-43648: CLONE - Multi-select customer attribute rendered in admin UI as a dropdown

app/code/Magento/Ui/Component/Form/Element/MultiSelect.php

+<?php
+/**
+ * Copyright © 2015 Magento. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\Ui\Component\Form\Element;
+
+class MultiSelect extends Select
+{
+    const NAME = 'multiselect';
+
+    const DEFAULT_SIZE = 6;
+
+    /**
+     * @inheritDoc
+     */
+    public function prepare()
+    {
+        $config['size'] = self::DEFAULT_SIZE;
+        $this->setData('config', array_replace_recursive((array)$this->getData('config'), $config));
+        parent::prepare();
+    }
+}

app/code/Magento/Ui/view/base/ui_component/etc/definition.xml

@@ -199,7 +199,7 @@
             </item>
         </argument>
     </select>
-    <multiselect class="Magento\Ui\Component\Form\Element\Select">
+    <multiselect class="Magento\Ui\Component\Form\Element\MultiSelect">
         <argument name="data" xsi:type="array">
             <item name="template" xsi:type="string">ui/form/element/multiselect</item>
             <item name="js_config" xsi:type="array">
@@ -209,6 +209,9 @@
                     <item name="elementTmpl" xsi:type="string">ui/form/element/multiselect</item>
                 </item>
             </item>
+            <item name="config" xsi:type="array">
+                <item name="size" xsi:type="string">6</item>
+            </item>
         </argument>
     </multiselect>
     <textarea class="Magento\Ui\Component\Form\Element\Textarea">