How can I set full width page layout in a default post page?

You have to edit your single.php for a full width design in your post view, is the single.php is responsible showing your post details.

Suppose, if your single.php looks like this:

+---------------------+
|                     |
+--------------+------+
|              |      |
|              |      |
|              |      |
|              |      |
|              |      |
+--------------+------+
+---------------------+

And your codes are like this:

<?php get_header(); ?>

<div id="content" class="single-content">
   CONTENT
<div>

<div id="sidebar">
   <?php get_sidebar(); ?>
</div>

<?php get_footer(); ?>

ACTION

Then to achieve the full width, you have to DELETE the sidebar div completely, and have to add a 100% width to the content like:

.single-content{width: 100% !important;}

This is just a concept. I hope you can find your solution from here.