By default, Atticus uses the Body Font for headings until you modify the Heading Settings in your theme. The default font sizes for each heading level are as follows:
H2: 1.2 times the size of the Body Font
H3: 1.15 times the size of the Body Font
H4: 1.12 times the size of the Body Font
H5: 1.09 times the size of the Body Font
H6: 1.07 times the size of the Body Font
If you prefer to keep the heading font consistent with the body font, you can simply leave the heading font on the 'default' setting.