Hello @caspiton,
Thanks for asking. 
There are couple of ways to setup fonts sizes. One is font manager wherein you get the option to select the font and save the changes so that it becomes a template. After that you can use the same font in various elements like headline or text element. Same elements present you the option to change font sizes for each individual elements. Second you can use custom CSS to set the font sizes as per your requirement.
Let me share both the solutions and you can use any of them as per your requirement.
- Font Manager: To help you get started, I have recorded a video screencast. Please take a look.

If you would like to explore font manager in more detail, please take a look at following article.
- You can add following CSS under X > Theme Options > CSS to change the font sizes:
h1, .h1 { font-size: 10px !important; }
h2, .h2 { font-size: 10px !important; }
h3, .h3 { font-size: 10px !important; }
h4, .h4 { font-size: 10px !important; }
h5, .h5 { font-size: 10px !important; }
Thanks.