the size of the gallery is set in the /galleries/grid_lightbox/css/common.css file
this style is setting the width to 728px:
font-family: Arial, Helvetica, sans-serif;
padding: 8px 0 8px 8px;
you an increase the galery width by editing the width setting of the .pg-gl selector.
the size of the thumbnails is set by the /galleries/grid_lightbox/index.php page. this line:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=132c&full=782p'));
sets the thumbnails to use the a32px cropped size.
see this thread for details on changing that and or creating custom image sizes:
For the error you are getting, here is an updated copy of the powergallery/admin_gallery/images/installed.png file.