Hi,
when the warning should be displayed? After you submit the form (but then it is actually not very useful, because the form has been saved) or dynamically during filling in the form?
As far as I know there is no way to display warning -- you can specify only a validator (which will throw an error). You can create your
own form control which will pass validation but will display the warning. Or you can simply use some javascript code to dynamically check length of the text and hide/display warning as the user types.