get ACF image size URL by ID

index.php

<?php

// first, get the image ID returned by ACF
$image_id = get_field('my_image_field');

// and the image size you want to return
$image_size = 'thumbnail';

// use wp_get_attachment_image_src to return an array containing the image
// we'll pass in the $image_id in the first parameter
// and the image size registered using add_image_size() in the second
$image_array = wp_get_attachment_image_src($image_id, $image_size);

// finally, extract and store the URL from $image_array
$image_url = $image_array[0];

?>

<img src="<?php echo $image_url; ?>" alt="">