function newWin(url,nwidth,nheight)
{
if (nwidth == null) {
nwidth = 440;
}
if (nheight == null) {
nheight = 300;
}
screenx = (screen.availWidth - nwidth) / 2;
screeny = (screen.availHeight - nheight) / 2;
var hWnd = window.open(url,"winLOV","toolbar=no,width="+nwidth+",height="+nheight+",left="+screenx+",top="+screeny+",resizable=0,scrollbars=yes,menubar=no,directories=no
var hWnd = window.open(url,"winLOV","toolbar=no,width="+nwidth+",height="+nheight+",left="+screenx+",top="+screeny+",resizable=0,scrollbars=yes,menubar=no,directories=no"

;
hWnd.focus();
}
Also you need to call the function with the window size and other stuffs like,
JavaScript:newWin("abc.jsp",600,375)
Enjoy all the Best
Baski.