How do I add the ability to change font size in CKEditor? I'm trying to install the CKEditor Font Size Module in my my editor. I installed the module via composer and then proceeded to copy the font/plugin.js file from the Font Plugin's page into libraries/ in my document root, per the instructions.

I also threw in a console.log() call into the font/plugin.js file to make sure it was loading. I modified the configuration of the Basic HTML editor to include the font size and family buttons in the toolbar on the /admin/config/content/formats/manage/basic_html page of my site. When I go to create a piece of content that uses the Text Editor, I don't see the font/size buttons I've configured.

Checking the Javascript console, I see my log message so I know the font/plugin.js file is being loaded. There are no error messages in the console. Why would the buttons not be showing up in this case?
Kevin's comment was spot on and it was in fact the Basic HTML configuration that was preventing the button from rendering. Specifically, the filter "Limit allowed HTML tags and correct faulty HTML" was the issue. The buttons rendered correctly in Full HTML mode so that was the hint.

In the filter configuration at the bottom of the /admin/config/content/formats/manage/basic_html page, I modified the allowed tags. By default the filter allows plain span tags without any attributes. However, this plugin uses span tags with inline style attributes. As a result, I had to change &lt;span&gt; to &lt;span style&gt; in Allow Tags list.

I can now see the font and size buttons in the editor, and using the tool injects HTML like the following:

&lt;p&gt;The &lt;span style="font-size:72px;"&gt;Quick red&lt;/span&gt; fox jumps over the lazy brown dog&lt;/p&gt;