What is the best way to force fixed size pop-up windows?

I'm trying to force the CVV help popup to a specific size on the checkout page. In Firefox at least, it appears too small and forces the user to resize or use the scrollbars.

I was under the impression the file to edit was this one:
/includes/modules/pages/checkout_payment/jscript_main.php

However, changing the dimensions has no effect...