To change size of Site Title in Squarespace, you can follow these.
#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
