How do I add a featured image to a single post page?

In single.php, find <?php the_content(); ?> around line ~17.
Just before that. paste following code.

<?php if ( has_post_thumbnail() ):?>
<div class="featured-image-wrap"><?php the_post_thumbnail(); ?></div>
<?php endif; ?>

This will show featured image if assigned to the post. To change the size of image you can pass parameter to the function the_post_thumbnail(). See official documentation of the_post_thumbnail(). I believe you would use child theme for modification.