Put the following three lines of code in your Joomla Template's Index.php file just above /header within script tag.


var aw1=(screen.availWidth)/4;
var aw2= aw1 * 3;
var marg=Math.round(Math.abs(aw1/2),0);


and add following code in attrib var at line # 371 of your helper.php file in modules/mod_mainmenu just before "toolbar=no" except double quotes

"width='+aw2+',left='+marg+',"

and your new window will open in center of the screen with 75% of the screen size.

must keep backup copy of your helper.php before updating codes.