This would probably just be a conflict between the template on your site and our CSS for SCLogin. Since your site is not live and accessible by us, you will have to figure out what CSS changes are required. Many browsers like Chrome and Firefox have Developer Tools where you can inspect and experiment with CSS. I would suggest tweaking some of the styles in there and/or try adding a height to that modal.
Once you figure out the correct CSS changes for the SCLogin, then I'd recommend making your own theme so that your changes don't get lost if you upgrade SCLogin. For information on how to do that, see
www.sourcecoast.com/sclogin/documentation/themes
Let me know if you have any questions going forward.
-Melissa