Is there a way to adjust the width of the left-side column of every SharePoint page?

There are 2-3 ways to change the width of Left Bar. You can make changes in Master page if you have access to it. Or you can write below code to your css file which is common for whole site:

#sideNavBox{width: 200px !important;}

Edit:

Apply CSS to whole site These are the steps: Step #1: Enable Publishing Infrastructure. Site Settings > Under Site Collection Administrator- Site Collection Features > Activate SharePoint Server Publishing Infrastructure

Step #2: Current Master Page: Site Settings > Under Look and Feel - Design Manager > On Left Side Panel - Publish and Apply Designs > Assign Master Pages to your site based on Device Chanel You will see Pop Up and there you will found Default Master Page. On scrolling down You will see Alternate CSS Url Click on that and You will be able to give url of your own css file which will work for whole site.

If you want to know more about how to create and to modify Master Pages click here