Check the width of a post's featured image in WordPress and use it to determine which image size should be displayed

single.php

<?php

function nd_featured_image_size_sniff() {

	if ( ! has_post_thumbnail() ) {
		return;
	}

	$featured_image_id = get_post_thumbnail_id( get_the_ID() );
	$featured_image_metadata = wp_get_attachment_metadata( $featured_image_id );

	if ( $featured_image_metadata['width'] > 819 ) {
		// Display large image size
	} else {
		// Display fallback smaller image size
	}

}