Thanks for the respond.
Unfortunately the video doesn't offer a solution, because, like mentioned in my post, I'm looking to change the width of my Blog Column and not the blog posts. The blog column is when you see an overview of the blog posts, before clicking to read one of them. In the video you linked to that page is shown at 0:09
It's this "grid" or in my case, list, I want to make narrower on my page.