functions.php

<?php

/**
 * Large Image for First Post
 *
 */
function be_blog_large_first_image( $image_size ) {
  global $wp_query;
  if( 0 == $wp_query->current_post && ! get_query_var( 'paged') )
    $image_size = 'large';
  return $image_size;
}
add_filter( 'genesis_pre_get_option_image_size', 'be_blog_large_first_image' );

/**
 * First Post Class
 * 
 */
function be_first_post_class( $classes ) {
  global $wp_query;
  if( 0 == $wp_query->current_post && ! get_query_var( 'paged' ) )
    $classes[] = 'first-post';
  return $classes;
}
add_filter( 'post_class', 'be_first_post_class' );
}