How To Make Lightbox Resize Equally When Being Loaded

I am making a simple lightbox without using the silly plugins and I want to know how to make the lightbox resize equally when its being loaded. I can only get it expand from the corner, I want it to stretch down then widen but equally on both sides.

This is the code im using:
// Javascript
<script language="javascript">
$(function(){
$("a#show-panel").click(function(){
$("#lightbox, #lightbox-panel").animate({"height" : "400px"},2000);
$("#lightbox, #lightbox-panel").animate({"width" : "400px"},2000);
});
$("a#close-panel").click(function(){
$("#lightbox, #lightbox-panel").fadeOut(300);
});
});
</script>

// CSS
#lightbox {
display:none;
background:#000000;
opacity:0.9;
filter:alpha(opacity=90);
position:absolute;
top:0px;
left:0px;
min-width:100%;
min-height:100%;
z-index:1000;

#lightbox-panel {
display:none;
position:fixed;
top:100px;
left:50%;
margin-left:-200px;
background:#FFFFFF;
padding:10px 15px 10px 15px;
border:2px solid #CCCCCC;
z-index:1001;
}

// HTML
<a id="show-panel" href="#" class="small">Show Panel</a>
<div id="lightbox-panel">
<h2>Lightbox Panel</h2>
<p>You can add any valid content here.</p>
<p align="center">
<a id="close-panel" href="#" class="small">Close this window</a>
</p>
</div><!-- /lightbox-panel -->
<div id="lightbox"> </div><!-- /lightbox -->