get ACF image size URL by image object

index.php

<?php

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

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

// now, we'll exctract the image URL from $image_object
$image_url = $image_object['sizes'][$image_size];

?>

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