add-param-to-show-number-of-posts-in-rss.php

<?php
/**
 * Add Param to Show Number of Posts in RSS
 *
 * Source: https://wordpress.stackexchange.com/questions/4736/get-all-posts-in-rss
 */
add_filter('option_posts_per_rss', 'my_posts_per_rss');
function my_posts_per_rss($option)
{
	if (isset($_GET['posts'])) {
		return (int) $_GET['posts'] ?: $option;
	}
	return $option;
}