Snippet for creating responsive images while controlling scrset values.

wp-custom-responsive-image-markup.php

<?php
$imageID = get_sub_field('img_id'); // ACF should return image ID
$size = 'full';
?>

<img
    class="jetpack-lazy-image"
    src="<?php echo wp_get_attachment_image_url( $imageID, '$size' ) ?>"
    srcset="<?php echo wp_get_attachment_image_srcset( $imageID, '$size' ) ?>"
	sizes=" (min-width: 1400px) 800px, 
            (min-width: 860px) 60vw, 
            100vw"
	alt="<?php echo get_post_meta($imageID, '_wp_attachment_image_alt', true); ?>"
/>