The easiest way I can say for you to do this is to add in some custom css:
position: absolute !important;
This may cause some z-indexing issues however, but it may be worth a quick try. I can see a usability issue in having a small screen and the popup being larger than the screen size... It doesn't make for a very nice experience in my opinion.
Because the wordpress forums seem to be a bit rubbish about informing me of new support posts, if you've any more issues in the future, please feel free to email me: firstname.lastname@example.org