Well, I have set the featured image or any other from post with this code:
Get the image
$image_id=get_post_thumbnail_id(get_the_ID());
$image_url = wp_get_attachment_image_src($image_id,'full');
$image_url=$image_url[0];
Set the image as background
<div id="full" class="full float-left" style="background-image:url(<?php get_image_url(); ?>);">
Your content here.
</div>
Next, one example I made few years ago taking the image from random post, the image is set as background and cover class. See sample
.full {
background-size: cover;
width: 100%;
}