Hi Ger, I was looking for a similar solution, not sure if it's what you want but I managed to make a 100% height and width RCS buy putting the HTML inside a 100% height row and column in RSD, then applying 100% height to every child element in the slideshow using an override css file.
Example here:
http://www.ptnsystems.com/yourwebsite/
Not sure how long this will be live though as I use this site for development purposes so here's some longhand css which may help explain:
body > .row.ROW-CLF.SLIDESHOW
{height: 100%;}
.html-element.slideshow-header
{height: 100%;}
.slideshow_container .slides_container
{height: 100%;}
.slideshow_container#slideshow-homepage .slideshow.slideshow-1
{height: 100%;}
.slideshow_container.slideshow-container-1#slideshow-homepage
{height: 100%;}
.slideshow_container#slideshow-homepage > .slideshow .slider-container.container-clf
{height: 100%;}
.slideshow_container#slideshow-homepage .slides_container > .slide.slide-1, .slideshow_container#slideshow-homepage .slides_container > .slide.slide-2, .slideshow_container#slideshow-homepage .slides_container > .slide.slide-3
{height: 100%;}