Setting the number of posts on Posts page for handhelds in Genesis.

functions.php

<?php
//* Do NOT include the opening php tag

add_action( 'pre_get_posts', 'sk_change_posts_per_page' );
/**
 * Set Posts Per Page on Handhelds (Tablets and Mobiles)
 *
 * Context: Posts page
 *
 * @author Sridhar Katakam
 * @link   http://sridharkatakam.com/
 */
function sk_change_posts_per_page( $query ) {

	if ( ! wp_is_mobile() ) {
		return;
	}

	if( $query->is_main_query() && !is_admin() && is_home() ) {
		$query->set( 'posts_per_page', '5' );
	}

}