Hi Paolo
To ensure your slideshow remains full width on all screen sizes try adding the following to the Custom CSS field of your templates parameters...
#container_slideshow .wrapper960 {
width: 100% !important;
}
.slidesjs-container, .slidesjs-control {
width: 100% !important;
}
On a side note I would strongly suggest converting your slideshow images to JPG as their current image file size is very big (your PNG images are 1Mb+ each). Your site looks great however your slideshow is taking a long time to load (over a minute on a slow to average connection).
CiarĂ¡n