<?php
// WP_Query arguments
$args = array (
    'posts_per_page'         => '5',
);
$counter = 0;

// The Query
$query = new WP_Query( $args );

// The Loop
if ( $query->have_posts() ) {
    while ( $query->have_posts() ) {
        $query->the_post();

        if($counter === 0) {
            echo '<div class="gross">';
            the_post_thumbnail( 'medium' );
            the_title();
            the_content();
        } else {
            echo '<div class="klein">';
            the_post_thumbnail( 'thumbnail' );
            the_title();
            the_excerpt();
        }
        echo '</div>';
    $counter++;
    }//end while
} else {
    echo 'Keine Beiträge gefunden';
}

// Restore original Post Data
wp_reset_postdata();