WordPress: custom number of posts for homepage only

gistfile1.php

<?php

/**
 * This piece of code let you use a different number of posts for the homepage, than the default posts_per_page setting.
 */ 

add_action( 'pre_get_posts', 'yourprefix_filter_posts' );
add_filter('found_posts', 'yourprefix_adjust_offset_pagination', 1, 2 );

$posts_on_homepage = 7; // Custom number of posts for the homepage
$default_per_page = get_option('posts_per_page');

/**
 * Alters the nuber of posts for the homepage
 */ 
function yourprefix_filter_posts( $query ) {
	if( is_admin() || false == $query->is_main_query()  ) {
		return;
	}

	if( false == $query->is_paged ) {
		$query->set( 'posts_per_page', $posts_on_homepage );
	}
	else {
		$page_offset = ( ( $query->query_vars['paged'] - 1 ) * $default_per_page ) - ( $default_per_page - $posts_on_homepage );
		$query->set('offset', $page_offset );
	}
}

/**
 * Alters the found_posts veriable, so the pagination will not break.
 */ 
function yourprefix_adjust_offset_pagination( $found_posts, $query ) {
	if( is_admin() || false == $query->is_main_query() ) {
		return;
	}

	// Define the homepage offset...
	$found_posts = ( $found_posts - $posts_on_homepage ) + $default_per_page;

	return $found_posts;
}