Random Size on Featured Image

You can put the sizes into an array and use PHP’s array_rand to choose a random size:

$sizes = array( 'small', 'medium', 'large' );

foreach( $pages as $page ){
    $random_size = array_rand( $sizes, 1 );
    echo get_the_post_thumbnail( $page->ID, $sizes[$random_size] );
}