#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