I give u a wider answer so u can play along with it:
1. open the file “style.css” in your theme (either locally and then ftp it or via the theme editor in the admin console)
2. all the typography is in the section “/* Begin Typography & Colors */” (for this specific theme)
3. Everywhere you see “font-size: 1.2em ” etc… means this influences the (relative in this case) size of the displayed elements as mentioned. Just play with the numbers until you like it.
On devguru: https://www.devguru.com/technologies/css2/, you can find a CSS reference, so you can quickly lookup what you can change. e.g. font-size is here: https://www.devguru.com/technologies/css2/8090.asp
If you want to have some more deduction capabilities of what is visible on your screen you can also try the web developer extension for firefox: https://chrispederick.com/work/webdeveloper/ , it shows you all the elements on a page and lots of stuff more. It even lets you change the html on the fly so you can see what it looks like immediately (ofcourse you then have to incorporate the changes you made later manually in your theme but it gives you a good idea). yes its free.
https://www.cogmios.nl
