In order to make the textarea resizable, the plugin wraps it in a div which takes on it's computed outerWidth and outerHeight (in pixels), regardless of what is set as width and height (in your case a percentage). A workaround is to add .parent().width('100%') after the .resizable() call. This would only let you leave the width as a percentage and not have the width be user-resizable (you would change handles to 's' instead of 's,e'). Resizables doesn't currently support percentage resizing, but you can request the feature by entering a feature/enhancement ticket here:
<a href="http://ui.jquery.com/bugs/newticket">
http://ui.jquery.com/bugs/newticket</a> (note: requires registration)
Also, in the future, should you have any other questions, there is a dedicated jQuery UI mailing list:
<a href="http://groups.google.com/group/jquery-ui/">
http://groups.google.com/group/jquery-ui/</a>- Richard