index.php?type=rss;action=.xml;sa=news;board=159.0;limit=25 is correct.
It does however apply a limiter to avoid performance trouble on your forum.
News.php:
$request = $smcFunc['db_query']('', '
SELECT num_posts
FROM {db_prefix}boards
WHERE id_board = {int:current_board}
LIMIT 1',
array(
'current_board' => $board,
)
);
list ($total_posts) = $smcFunc['db_fetch_row']($request);
$smcFunc['db_free_result']($request);
$feed_title = ' - ' . strip_tags($board_info['name']);
$query_this_board = 'b.id_board = ' . $board;
// Try to look through just a few messages, if at all possible.
if ($total_posts > 80 && $total_posts > $modSettings['totalMessages'] / 10)
$context['optimize_msg']['lowest'] = 'm.id_msg >= ' . max(0, $modSettings['maxMsgID'] - 600 - $_GET['limit'] * 5);
That entire block should probably be removed.