RESOURCE LIBRARY EXAMPLE:

HTML:
<div class="sort">
       <a href="/resources/?sort=alpha">alpha</a> |
       <a href="/resources/?sort=recent">recent</a> |
       <a href="/resources/?sort=oldest">oldest</a> 
</div>

PHP:
<?php
wp_reset_query();
$args = array(
    'post_type' => 'resource',
    'posts_per_page' => -1,
);

if (isset($_GET['sort']))
{
    if ('alpha' == $_GET['sort'])
    {
        $args['orderby'] = 'title';
        $args['order'] = 'ASC';
    }
    
    elseif ('oldest' == $_GET['sort'])
    {
        $args['order'] = 'ASC';
        $args['orderby'] = 'date';
    }

    elseif ('recent' == $_GET['sort'])
    {
        $args['order'] = 'DESC';  
        $args['orderby'] = 'date';
    }
}

query_posts($args);
?>

In your case you could make the sort links (the HTML provided) into numbers for posts per page.

SORT BY NUMBER OF POSTS:

EXAMPLE:

HTML:
<div class="sort">
       <a href="/resources/?sort=25">25</a> |
       <a href="/resources/?sort=50">50</a> |
       <a href="/resources/?sort=100">100</a> |
       <a href="/resources/?sort=all">All</a>
</div>

 Then for your args you could do something like the following (I only did 2 of them):

PHP:
<?php
wp_reset_query();
$args = array(
    'posts_per_page' => -1,
    'orderby' => DESC,
);

if (isset($_GET['sort']))
{
 if ('25' == $_GET['sort'])
    {
        $args['posts_per_page'] = '25';
    }
elseif ('all' == $_GET['sort'])
    {
       $args['posts_per_page'] = '99999999';
     }
}

query_posts($args);
?>