Theme editing “post thumbnail” help

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%;
}