You could use viewport-percentage lenghts.
See: http://stanhub.com/how-to-make-div-element-100-height-of-browser-window-using-css-only/
It works like this:
.element{
height: 100vh; /* For 100% screen height */
width: 100vw; /* For 100% screen width */
}
More info also available through Mozilla Developer Network and W3C.