All your server  code does it send it to the browser.

To open  a new window  that side you HAVE to tell it to do it from javascript.

There are ways to do it so it "looks" like server code  is doing it.

You can write a server control  that outputs the required javascript along with the url.

Or you can Add attributes to a server control or an existing "off the shelf" control.

But all in all - IT HAS TO BE JAVASCRIPT that does the deed.

At teh end of the day you might as well turn your script below into a function and manually put it at teh top of you aspx  page - calling via hrefs.

<script>
function openit(mypage){
"window.open 
(mypage,null,"height=200,width=400,status=yes,toolbar=no,menu
bar=no,location=no");"
}
</script>

called via hrefs like
<a href=javascript:void(0) onclick=openit("anypage.aspx")>OpenIt</a>
 or
<a href=javascript:openit("anypage.aspx")>OpenIt</a>

You can accomplish this by adding an attribute to your DropDownList.
This will add a Javascript event handler to your rendered HTML and
allow you to easily open  a new window:

In the Page_Load event add this code

DropDownList1.Attributes.Add("onchange", "window.open 
('TestPage.aspx','myNewWindow','height=200,width=400,status=yes,toolbar=no,menu
bar=no,location=no'))";

I knew I could add an attribute, but I didn't know it was the "onchange" one for
a dropdown list. I'll try it soon.

In your codebehind, you should inser some client javascript to Response... Or to
same placeholder that will execute when the page gets loaded on the client...