To Use Gallery Module:

  <script type="text/javascript" src="http://yui.yahooapis.com/3.5.1/build/yui/yui-min.js"></script>
  <script type="text/javascript">
    YUI ({
      gallery: "gallery-2012.05.09-20-27"
    }).use ("gallery-textarea-autoheight", function (Y) {
      Y.one("textarea").plug (Y.TextareaAutoheight);
    });
  </script>

To Use Downloaded Code:

  <script type="text/javascript" src="http://yui.yahooapis.com/3.5.1/build/yui/yui-min.js"></script>
  <script type="text/javascript" src="textareaAutoheight-min.js"></script>
  <script type="text/javascript">
    YUI ().use ("textareaAutoheight", function (Y) {
      Y.one("textarea").plug (Y.TextareaAutoheight);
    });
  </script>

* If you pass maxRows parameter, it will add scrollbar after those many number of rows.

Y.one("textarea").plug (Y.TextareaAutoheight, {maxRows: 4});

* To adjust height manually:

Y.one("textarea").textareaAutoheight.adjustHeight();