在循环中时，可以使用current_post 方法你没有发布任何代码，所以我只想做一个简单的演示。

If ( have_posts() ) {
    while( have_posts() ){
        the_post();

        // Setting the global $wp_query to access the method
        global $wp_query;

        // Get the current post\'s index
        $current_post = $wp_query->current_post;

        // Let\'s calculate the thumbnail size based on the current post\'s index
        switch ( $current_post ) {
            case 1 :
                $size = \'size-1\';
                break;
            case 2 :
                $size = \'size-2\';
                break;
            default :
                $size = \'post-thumbnail\';
                break;
        }

        // Now we have the proper thumbnail size
        the_post_thumbnail ( $size );

    }
}

您可以尝试以下代码。这是杰克·约翰逊发布的代码的更新版本。

if ( have_posts() ) {
    global $wp_query;
    $thumbnail_size = \'size-after-3-posts\';

    while ( have_posts() ) {
        the_post();
        // Get the current post\'s index
        $current_post = $wp_query->current_post;

        // Post index starts from 0 (zero)
        // 1st post will have big-thumbnail and 2nd & 3rd will have small-thumbnail
        if ( 0 === $current_post ) {
            $thumbnail_size = \'big-thumbnail\';
        } elseif ( 1 === $current_post || 2 === $current_post ) {
            $thumbnail_size = \'small-thumbnail\';
        }

        // Now we have the proper thumbnail size
        the_post_thumbnail( $thumbnail_size );
    }
}