Get the image URL from ACF, including choice of dimension (from add_image_size())

acf_get_image.php

/**
 * Get ACF Image
 *
 * @param  string   $size    – the dimension of the image (using add_image_size() function)
 * @param  string   $key     – the ACF field key
 * @param  string   $post_id – the ID of the post (if empty, will use $post->ID)
 * @return string   The image URL
**/
function get_acf_image( $size = 'full', $key = 'image', $post_id = '' ) {

	if($post_id == ''):
		global $post;
		$post_id = $post->ID;
	endif;

	$image = get_field($key, $post_id);

	if( !empty($image) ):
		if($size != 'full'):
			$url = $image['sizes'][$size];

			if($url == ''):
				$url = $image['url'];
			endif;
		else:
			$url = $image['url'];
		endif;

		return $url;
	else:
		return null;
	endif;
}