Hi
Looked into this a little bit more
If TinyMce editor is used then it might be achievable as this editor has a pluin feature
One of these is a character word count
Here is link to techy info,
archive.tinymce.com/wiki.php/Tutorials:Creating_a_plugin A bit beyond my knowledge to implement into post ad description area
Maybe this will be of help to dev
Regards
Iain