you need to edit the line in the index file to use a smaller gallery size:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782p'));
if 782px is too small, you will need to edit the admin/common/properties.php file to define a new galley size.
see the following power gallery thread for details:
showthread.php?t=10969