You can make your blog posts more narrow or wider at any time. You can also change the width of the sidebar too. Using a combination of these options, you have complete control over the size/width of your content.
To change the width of your entire website, follow the steps below:
1) Go to Appearance > Customize in your dashboard:
2) Look for the "Layout Options" tab on the left:
3) In the options available, you can set the main width of your website by using the option shown below:
4) Once you've changed the main width of your website, you might like to increase/reduce the size of the main blog post column. The main blog post column is the space that contains your posts on the homepage (when the sidebar is active). Any remaining space is taken up by the sidebar:
The video below shows a quick example of both features in action:
Article tags: adjust width, widths, size, side bar