To do this you have to use JavaScript.
Dreamweaver doesn't compile very sound JS in my experience but those nice guys at internet.com (Eric King wrote this paticular script) have the source there for you to download for free...
<!-- TWO STEPS TO INSTALL CENTERED POPUP:
1. Copy the coding into the HEAD of your HTML document
2. Add the last code into the BODY of your HTML document -->
<!-- STEP ONE: Paste this code into the HEAD of your HTML document -->
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
<!-- Original: Eric King (eric_andrew_king@hotmail.com) -->
<!-- Web Site:
-->
<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!!
-->
<!-- Begin
function NewWindow(mypage, myname, w, h, scroll) {
var winl = (screen.width - w) / 2;
var wint = (screen.height - h) / 2;
winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',scrollbars='+scroll+',resizable'
win = window.open(mypage, myname, winprops)
if (parseInt(navigator.appVersion) >= 4) { win.window.focus(); }
}
// End -->
</script>
</HEAD>
<!-- STEP TWO: Copy this code into the BODY of your HTML document -->
<BODY>
<!-- Put the URL inside the link's href="..." section, then carefully -->
<!-- change 'name' to a name for your popup window. The last three -->
<!-- items represent the popup window width, height, and scrollbars. -->
<a href="
onclick="NewWindow(this.href,'name','400','400','yes');return false;">Popup Yahoo.com</a>
<p><center>
<font face="arial, helvetica" size="-2">Free JavaScripts provided<br>
by <a href="
JavaScript Source</a></font>
</center><p>
<!-- Script Size: 1.12 KB -->