如何为ACF库创建自己的自定义图像大小

I'm using Advanced Custom Fields to create an image gallery. At the moment I have the image size set to thumbnail I tried setting it to medium but it didn't work so I don't know if I'm missing something. But I would like to create my own image size anyway. So how would I do this? I was looking on the ACF site but it seems to have gone down.

<div class="container">
    <div class="row">
        <div class="col-md-12">
    <?php 

    $image_ids = get_field('gallery', false, false);

    $shortcode = '[gallery ids="' . implode(',', $image_ids) . '"]';

    echo do_shortcode( $shortcode );

    ?>

    <?php 

    $images = get_field('gallery');

    if( $images ): ?>

        <ul>
            <?php foreach( $images as $gallery_images ): ?>
                <li>
                    <a href="<?php echo $gallery_images['url']; ?>">
                         <img src="<?php echo $gallery_images['sizes']['thumb-nail']; ?>" alt="<?php echo $gallery_images['alt']; ?>" />
                    </a>
                    <p><?php echo $gallery_images['caption']; ?></p>
                </li>
            <?php endforeach; ?>
        </ul>
    <?php endif; ?>
        </div>
    </div>
</div>

Generally, thumbnail is written 'thumbnail' and not thumb-nail. Maybe this is it ?

Also, to create a custom image size in Wordpress, you need to add it to your functions.php file. Something like add_image_size( 'my-new-size', 220, 180, true );

see https://developer.wordpress.org/reference/functions/add_image_size/

<?php 
            $images = get_field('immagine');
            if( $images ): ?>
            <?php foreach( $images as $image ): ?>
                <li><img src="<?php echo $image['sizes']['large']; ?>" alt="<?php echo $image['alt']; ?>"></li>
             <?php endforeach; ?>
             <?php endif; ?>