Ciao usa questo script

codice:

<script language="JavaScript">
<!--
function NewWin(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=yes'
win = window.open(mypage, myname, winprops)
if (parseInt(navigator.appVersion) >= 4) { win.window.focus(); }
}
-->
</script>

Poi il collegamento lo fai così:

<a href="javascript:NewWin('_non_letti.php','','170', '150','yes')">