When using Advanced Custom Field plugin for WordPress, this allows you to target custom image sizes

acf_image.php

<?php
  $attachment_id = get_field('image');
  $size = "square"; 
  $image = wp_get_attachment_image_src( $attachment_id, $size );
?>
<img alt="Image of <?php the_title(); ?>" src="<?php echo $image[0]; ?>">

// IF GROUP
<?php
  $attachment_id = get_sub_field('image');
  $size = "square"; 
  $image = wp_get_attachment_image_src( $attachment_id, $size );
?>
<img alt="Image of <?php the_title(); ?>" src="<?php echo $image[0]; ?>">

// IMAGE ARRAY
<?php
  $image = get_sub_field('image');
  $image_url = $image['sizes']['slider'];
  $size = "slider"; 
?>
<img src="<?php echo $image_url; ?>" alt="<?php echo esc_attr($image['alt']); ?>">