There is no limitation for the content of news
In my deployed Redmine, I tried to create a news with more than 65536 characters. And I got this as attached. There should be a constraint in new.rb, such as:
validates_length_of :description, maximum: 65535
even more, in the news/_form.html,
maxlength => 65535should be added to the textarea. I have attached the patch.