The body tag is set to height: 100%, then the first pane is also set to height: 100%, then the more dingus is set to position: absolute, bottom: 0, left: 50%, left-margin: (50% of dingus width). Background images can be specified with background-size: cover, which gets you the resizing behavior automatically.
It’s not easy to duplicate in Freeway. I did just such a page recently: http://www.definingmoments.me and I had to write an Action to get Freeway to even publish it the way I needed. It’s furiously non-WYSIWYG in the layout view, too, which causes my client no end of pain when he tries to edit it. (Not proud about that part.)
Publish the site somewhere, where we can see the HTML and CSS. Did you apply the PageDiv Extended Action to the page, and set the height to 100% there? Also did you set the height of the body to 100%. Both of those are required before you can get much further. Then you have to build your page as an inline layout, and set your first pane to a min-height of 100%.