正如@MridulAggarwal所言，这是您面临的一项非常基本的PHP任务：

$wpse69584_posts = get_posts( array( /* Your Arguments */ ) );

echo $post[0][\'post_title\'];
echo isset( $post[0]->post_excerpt ) 
    ? apply_filters( \'the_excerpt\', $post[0]->post_excerpt )
    : apply_filters( \'the_content\', $post[0]->post_content )
;
// etc.

global $wpdb;
if ( have_posts() )
{
    while( have_posts() )
    {
        the_post();

        if ( 1 <= $wpdb->current_post )
        {
             $thumb_size = \'BIG\';
        }
        // Add \'MEDIUM\' to 2nd post and \'SMALL\' to each other
        else
        {
             $thumb_size = ( 2 == $wpdb->current_post ? \'MEDIUM\' : \'SMALL\' );
        }

        echo \'<article \'.get_post_class( "post-number-{$wpdb->current_post}" ).\'>\';
            echo "<h{$wpdb->current_post}>";
                the_title();
            echo "</h{$wpdb->current_post}>";

            the_content();

            the_post_thumbnail( get_the_ID(), $thumb_size );
        echo \'</article>\';
    }
}