#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, click on the Custom of the Header Button
#5. Next, click Paragraph 2 in the Size option
#6. Choose Custom
#7. Change to the custom size you want and click Save on the left of the Header
