[Plugin: WP-PageNavi] Display different quantities of post per page

I’ve got the WP-PageNavi plugin working fine, but had a question about modifying how many posts are displayed per page.

Here is the loop I’m using.

<?php
          $args = array(
            'post_type' => 'commercial,
            'posts_per_page' => 4,
            'paged' => get_query_var('paged')
          );

          $my_query = new WP_Query( $args );

          while ( $my_query->have_posts() ) : $my_query->the_post();
        ?>
         <!-- post info here -->

        <?php endwhile; ?>

My overall goal is to display 2 posts on the first page and 8 posts on any of the subsequent paginated pages. I’ve use the if_paged() method to modify the posts_per_page variable, but the pagination gets messed up. Has anyone had any results with something like this?

http://wordpress.org/extend/plugins/wp-pagenavi/