Customize image size used in the loop by Genesis

gistfile1.php

<?php
/**
 * Customize image size used in the loop by Genesis
 *
 * @author Jared Atchison
 * @link http://jaredatchison.com/code/
 * @param string $image_size
 * @global array $wp_query
 * @global int $loop_counter
 * @return string
 */
function ja_custom_loop_image_size( $image_size ) {

	global $wp_query;
	global $loop_counter;
		
	$page = get_query_var( 'paged' );

	// If we are the first page of a post archive page, use the the 
	// 'featured' image size for the first post
	if ( $loop_counter < 1 && $paged < 2 && get_post_type() == 'post' )
		$image_size = 'featured';

	// Use 'portfolio' image size on portfolio archive pages
	if ( is_post_type_archive( 'portfolio' ) )
		$image_size = 'portfolio-small';

	return $image_size;
}
add_filter( 'genesis_pre_get_option_image_size', 'ja_custom_loop_image_size' );