Looks like you got the child theme set up -- which is excellent. Unless the theme has built-in options for specific font sizes on specific elements, no, you'll need to learn a bit about using CSS.
The best tool to start with is downloading Firebug - an addon for Firefox browser. It's super helpful as it shows you the CSS code setting the font size for an element on the page -- then you can use that code by copying it into your child theme and changing the font size there.
This is also a good reference for CSS: http://www.w3schools.com/css/
What specific font size do you want to change?