The formatting options available in Atticus primarily focus on global settings in the theme which are applied across the entire document.
However, you can use subheadings to change the appearance of specific segments of text. You can add subheadings to a paragraph by highlighting the section and using the H2-H6 options under the paragraph menu in the toolbar.
The size and font for subheadings can be adjusted in the theme settings, but subheadings will always appear slightly bolded, non-indented, and include extra padding.
However, beyond subheadings, you cannot adjust the size or font of individual sections of text within Atticus.