To adjust my WordPress sidebar font size, I had studied a day for CSS.

First, go to ‘Appearance -> Customize’ in ‘WP-admin’ page

Then Click ‘Additional CSS’

Type your codes like below. you can see the result in real time.

Copy to Clipboard

If you want to adjust more options in your theme, Press F12 to open Devtools in Chrome.

Then, choose ‘select an element’

When you point your mouse cursor on a specific area, you can see debugged code.

 For More Detailed Information. Always thanks for your contributions.