If there aren’t sections in the dashboard supporting these changes, you may need to use a Child Theme or Custom CSS plugin, otherwise you’ll be changing the theme’s files. Changes to theme files will erase once the theme updates.
Which text do you want changed in size, and how much bigger/smaller?
Try using a browser Inspector tool, such as Google Chrome’s built in Inspect Element feature or Firefox’s Firebug tool to find particular CSS styles in web pages.