With the child theme you actually get more control, but it means more hands on….although a great learning experience. The code would go into the child theme’s style.css file by going to Appearance >> Edit
It should be working because I just now tried it in my local test site and activated the child theme then added that font-size….perhaps clear your browser cache and see if that helps.
I have another question related to font size. I increased the overall font size but I was wondering if it was possible to increase the font size only for some pages. There are some things that I want larger than others but I don’t want to use headings.