var userWidth = screen.availWidth;

    var userHeight = screen.availHeight;

    var leftPos;
    var topPos;
    var popW = 860;   //set width here
    var popH = 620;   //set height here

    var settings = 'modal,scrollBars=no,resizable=no,toolbar=no,menubar=no,location=no,directories=no,';

    leftPos = (userWidth - popW) / 2,
    topPos = (userHeight - popH) / 2;
    settings += 'left=' + leftPos + ',top=' + topPos + ',width=' + popW + ', height=' + popH + '';
    
    window.open('popup.aspx', 'childForm', settings);