Index: src/wp-admin/css/edit.css
===================================================================
--- src/wp-admin/css/edit.css	(revision 52647)
+++ src/wp-admin/css/edit.css	(working copy)
@@ -1085,6 +1085,12 @@
 	margin: 8px;
 }
 
+#postcustomstuff #ajax-response input[type=text],
+#postcustomstuff #ajax-response select,
+#postcustomstuff #ajax-response textarea {
+	width: 96%;
+}
+
 #side-sortables #postcustomstuff table input,
 #side-sortables #postcustomstuff table select,
 #side-sortables #postcustomstuff table textarea {