in the galleries/slideshow_dissolve/index.php file, the following line sets the width of the large images to use, and this used to set the width of the gallery:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782c'));
this is setting the large image width to 782px
See the following thread for more details and instructions for changing the large image size. Changing the large image size will change the gallery size
showthread.php?t=8844&highlight=properties