You might be aware that maxlength attribute of textarea doesn't work for IE8 & IE9. If you still want to restrict the length then you can use below code using alloyUI :
A.one(document).delegate(['keypress','change'],function(e){
var maxLength = this.attr('maxlength');
if(this.val().length > maxLength) {
this.val(this.val().substring(0, maxLength));
e.preventDefault();
}
}, '#<portlet:namespace/>description');