Find and edit the javascript function named popupWindow so it looks more or less like this...
Code:
function popupWindow(url) {
window.open(url,'popupWindow','toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,copyhistory=no,width=600,height=480,screenX=150,screenY=150,top=150,left=150')
}
Adjust the width and height to suit.
The default ZC installation uses 100 and 100 for those two settings. Of course, that's reasonable for a default install, but could maybe be bumped up a bit so fewer ZC sites would have invisible larger images. 
HTH
Rob