This is a useful little snippet of code I got from ajaxray.com.  It shows the characters left in a HTML Textarea.  Much of the functionality can be customizable if needed.  Also, for the sake of security, you will want to check the characters submitted on the server side also since jQuery only checks on the client side.  Note:  You'll need to include your jquery files to use this.

In the head of the page do the following:{codecitation class="brush:javascript"}
<script language="javascript">
function limitChars(textid, limit, infodiv)
{
    var text = $('#'+textid).val();   
    var textlength = text.length;
    if(textlength > limit)
    {
        $('#' + infodiv).html(''+limit+' character limit reached');
        $('#'+textid).val(text.substr(0,limit));
        return false;
    }
    else
    {
        $('#' + infodiv).html(''+ (limit - textlength) +' characters left');
        return true;
    }
}
 
$(function(){
     $('#purpose').keyup(function(){
         limitChars('purpose', 250, 'charlimitinfo');
     })
});

</script>
{/codecitation}

In the body of the page do the following:{codecitation class="brush:html"}

<strong>Purpose:</strong><br /> <textarea name="purpose" id="purpose" rows="3" cols="25"></textarea><br />
    <span id="charlimitinfo" style="float:left; color:#aa3333; font-size:10px;" >250 characters left</span>

{/codecitation}