How do I add the ability to change font size in CKEditor?
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;