TinyMCE Advanced is a free plugin that lets you choose your preferred font size easily. This plugin provides you with a classic editor toolbar on the new Gutenberg editor.

First, you need to install and activate the plugin. Then, you can go to the Settings page. You’ll find two tabs: Block Editor (Gutenberg) and Classic Editor (TinyMCE).

If you want to use the classic editor only, enable Replace the Block Editor with the Classic Editor under the Advanced Settings. But, leave it unticked to use both editors side by side.

Here are the steps to change the font using this plugin:

1. Go create a new post or edit an existing one. You’ll see a classic paragraph editor.
   
2. Highlight the lines of text you want to change, then choose the font size.
   
3. The result will look like this: