There is literally no feature I would like to see incorporated in Muse more than support for 100% height elements. Just like there is a "stretch to page width" resize option, I'd really love to have that option for vertical elements. Muse is a design-centric application, and having 100% height elements (without making me go pay and download a stupid widget) is critical to achieving a responsive design.
In the meantime, what I'm forced to do is create a breakpoint for every screen width size, and then adjust elements according to which screen height I assume corresponds. For example: I know that a 1280px page width is often paired with a 800px page height: as a result, I divide my "sections" into 800px blocks and then adjust the content accordingly. But I really would like to see a better way of doing this in the future.