what controls the size of the image on the slideshow:
There is a slider at the bottom right of the Showcase window that can adjust the Thumbnail size or the Large Image size depending which you have selected
how can I make it cover the slideshow cover the whole screen
You cant because you have no way of knowing the viewers screen size when you create the Showcase slideshow
If you can tell us a bit more about what you are trying to do and can post a link to a page where you want this to happen we can probably offer more help.