#1. First, you click on the Site Styles icon on the right of the Header

#2. Next, you choose Fonts

#3. Scroll down to the bottom and choose Assign Styles

#4. Next, you click on the Heading of Site Title 

#5. Next, click Heading 2 in the Size option

#6. Choose Custom

#7. You can enter a custom size and click Save on the left of the Header