Somewhere in materialize.min.css the slider class is getting its height assigned. I am simply not able to override this.
What I tried is to set the height in the constructor of my widget:
public class HomeViewImpl extends Composite implements HomeView {
    public HomeViewImpl() {
        initWidget(uiBinder.createAndBindUi(this));     
        int height = Window.getClientHeight();
        slider.getElement().getStyle().setHeight(height, Unit.PX);
    }
}
I also tried to overwrite it in my own css file and I also tried to override Widget.onLoad() with the same result. I can resize the slider afterwards but not on when its actually loaded. Since I want it to use up the entire available space it has to be resized "on load".
Please note that setting fullscreen="true" is not an option since this would mess up the arrangement of my parallax effect I am using here as well.
<m:MaterialSlider ui:field="slider" fullscreen="false">
    <m:MaterialSlideItem height="100%">
        <m:MaterialImage url="http://mayastepien.nl/googlecalendar/google-drinks.jpg" />
        <m:MaterialSlideCaption textAlign="CENTER">
            <m:MaterialTitle title="This is our big Tagline" description="Here's our small slogan." />
        </m:MaterialSlideCaption>
    </m:MaterialSlideItem>
    <m:MaterialSlideItem height="100%">
        <m:MaterialImage url="http://dreamatico.com/data_images/car/car-1.jpg" />
        <m:MaterialSlideCaption textAlign="CENTER">
            <m:MaterialTitle title="This is our big Tagline" description="Here's our small slogan." />
        </m:MaterialSlideCaption>
    </m:MaterialSlideItem>
</m:MaterialSlider>
 
     
    