<?php
// first, get the image object returned by ACF
$banner_object = get_field('banner_background_image');
// and the image size you want to return
$banner_size = 'banner';
// now, we'll exctract the image URL from $image_object
$banner_url = $banner_object['sizes'][$banner_size];
?>
<!--then output the url of the thumb size as defined-->
<img src="<?php echo $banner_url; ?>" />