Just been playing around with this.
If you want to do it like the stackexchange site, that the user can select how many search results they wish to be displayed on a page, you could add a drop-down menu (or similar) to the WordPress search form.
The value that the user selects can then be picked up in the $_GET superglobal in functions.php
You could then write:
$wp_query->query_vars['posts_per_page'] = $_GET["delimit"];
where delimit is the name you gave your drop-down.