limit_chars.js

function limitChars(textarea, limit, infodiv) {
  $(textarea).observe("keyup", function(event) {
    var text = $F(event.element()); 
    var textlength = text.length;
    var info = $(infodiv);

    if(textlength > limit) {
      info.innerHTML = 'You cannot write more then ' + limit + ' characters!';
      event.element().setValue(text.substr(0, limit));
      event.stop();
    }
    else {
      info.innerHTML = 'You have '+ (limit - textlength) +' characters left.';
    }
  });

  $(textarea).observe("keydown", function(event) {
    if($F(event.element()).length == limit) {
      var key = event.which || event.keyCode;
      switch(key) {
        case Event.KEY_BACKSPACE:
        case Event.KEY_TAB:
        case Event.KEY_RETURN:
        case Event.KEY_ESC:
        case Event.KEY_LEFT:
        case Event.KEY_UP:
        case Event.KEY_RIGHT:
        case Event.KEY_DOWN:
        case Event.KEY_DELETE:
          break;
        default:
          if(!event.ctrlKey && !event.altKey && !event.metaKey) {
            event.stop();
          }
          break;
      }
    }
  });
}

// usage:
// limitChars("textarea_id", 190, "information_div_id");