It can be done, but it gets a little tricky since the html editor allows html, so it may seem to use more characters than you can see.
<span style="font-size:12px;">hi</span>
That will appear to be 2 characters if you are reading it, but it is actually almost 40 characters when you include the html.
You can use javascript to get the number of characters and display it on the page. You can google ckeditor character count, or look in the ckeditor documentation for how to get that value: https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_editor.html#method-getData
Our jquery validation allows you to set a max and min number of characters and prevent the form from submitting if it doesn't meet your requirements.
This is something I could help with in a premier support ticket, but I don't have a specific UI for it or tutorial.