To edit Navigation Bar (Size, Color) in Squarespace, you can follow these.
Edit Size
#1. First, click on the Site Styles icon
#2. Next, choose FONTS
#3. Click on the blank space in the Navigation bar
#4. The right side will open a new box, scroll down to the bottom and change the size you want
Edit Color
#1. First, click on the Site Styles icon
#2. Choose COLORS
#3. Click on your page color theme (in my example, it is Lightest 1)
#4. Click on Navigation Links to change the Navigation bar color