There's some information about how to work with responsive height documents in the 'responsive document tips' section here: Tumult Hype Animations Wordpress Plugin
For your case, adding an additional CSS class would help you set a minimum height for your document.
This may also be helpful: Creating a Flexible Tumult Hype Document within a DIV with no set 'height' if you can add the .js code to the 'head' of your post or page.