fix(FormGroup): size were invalid since default has been removed

Bug introduced in c59595f2c6

docs/content/3.forms/9.form-group.md

@@ -160,6 +160,10 @@ code: >-
 :u-input{placeholder="you@example.com" icon="i-heroicons-envelope"}
 ::
 
+::callout{icon="i-heroicons-exclamation-triangle"}
+  This will only work with form elements that support the `size` prop.
+::
+
 ## Props
 
 :component-props