You could do this by adjusting the spry (client side) validation.
In Dreamweaver design view, hover over the text area and click the blue 'Spry Textarea: xxxxxxx_spry' at the top of that area.
You should see the properties for the spry textarea in the properties panel/inspector. There is a box where you can specify Max chars, block extra characters, have it show the number of characters remaining, and provide a 'hint' to the end used - i.e. 'Max Chars 500'
EDIT: - Just noticed the HTML Editor bit. Not sure if it can be done when HTMLEditor replaces textarea.