Multiple item layout with one query

As @MridulAggarwal already stated, it’s a pretty basic PHP task that you’re confronted with:

$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.

Or if you’re using a loop, it’s even easier:

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>';
    }
}

Note, that this won’t work with a larger number as 6, as $wpdb->current_post will still increment, but there’s simply no <h7> and following 😉