How to make Imagefield slideshow display full width
I have not used the field slideshow module, but if it allows you to select the image style, then you could use the Lightbox2 module http://drupal.org/project/lightbox2. Lightbox 2 is not the only option for displaying the image in an overlay/popup type thing, there are many, here is a comparison of some of them http://drupal.org/node/266126. I just mention lightbox2 specifically as it is the one I usually use.