<form name = "myform" action="comment_post.php" method="POST" onsubmit = "return validate()> 

<script type = "text/javascript">

function validate() {
var x = document.myform.com_title.value;
if (x.length <  5) {  // or whatever minimum length is sensible
alert ("You must enter a title!");
document.myform.com_title.value = "";  // clear the field
document.myform.com_title.focus();  // and focus on it
return false;
</script>