the size of the gallery is set in the in the galleries/slideshow_wide/index.php file.
the following line:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=995c'));
sets the gallery to use the 995px cropped image size.
the available options are:
so you could change that line to use the 782px croped image size:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782c'));
if you wanted to create a custom width, you would need to edit the admin/common/properties.php file.
see the following power gallery thread for details: