The blank space is likely caused by an extension or your custom CSS. Increasing the number of preloaded discussions is unlikely to fix this issue, it will probably just push the blank space further down.
Try disabling any CSS or extension related to the footer feature and see if the problem disappears. Then re-enable them one by one to identify which one is responsible.
If your website is on the internet, you can share a link and we might be able to figure out which CSS rule is responsible, and work back to find its source from there.
If you really want to change the number of discussions per page, I don't think there is any extension for that. You would have to use the PHP extension API and change the default limit property on the DiscussionListController. Looking through the source code I think there will be an extender for this in 2.0. In 1.x you might need to change a static property on the class.