adjusting the height and width of innerHTML

I am a newbie on javascript, and here is a question I'd like to ask.
Thanks in advance for the help. Here you can see my point: right now,
I can ask the div of 'list_comment' to be 250x200. But when
'comments_display.php' is larger than 250x200, I really want a
scrollable list_comment, so that I can scroll down the comment in the
innerHTML.

Thanks,

<script>
function TurnOnDiv(e, div_name) {
theObject = document.getElementById(div_name);
theObject.style.width = "250px";
theObject.style.height = "200px";
var posx, posy;
if (!e) var e = window.event;
if (e.pageX || e.pageY) {
posx = e.pageX - 250;
posy = e.pageY - 200;
}
else if (e.clientX || e.clientY) {
posx = e.clientX + document.body.scrollLeft +
document.documentElement.scrollLeft - 250;
posy = e.clientY + document.body.scrollTop +
document.documentElement.scrollTop - 200;
}

theObject.style.left = posx + "px";
theObject.style.top = posy + "px";
theObject.style.visibility = "visible";
}

function ProcessAjax(serverPage, objID) {
xmlhttp = GetXmlHttpObject();
var obj = document.getElementById(objID);
xmlhttp.open("GET", serverPage);
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
obj.innerHTML = xmlhttp.responseText;
}
}
xmlhttp.send(null);
}


</script>

<div id="list_comment" style="visibility: hidden"</div>

<p style="font-size: 13px" <a href="javascript://"
onclick="TurnOnDiv(event, 'list_comment');
ProcessAjax('comments_display.php', 'list_comment')">Read Comments