Editing body/paragraph text size

Body text is set in the theme, and that's the best place to make adjustments. In the Theme you set the font size, which becomes the default for the body font. The other Theme Formats (Title, Subtitle, H1, H2, etc.) that are available on the Format tab in the Type section are then calculated as factors of the body font setting. For example, Title is 5X plus bold, if I recall correctly.

To find the font setting when editing a theme, it's on the Styles tab >> Typography section, and it's the first text input box you'll see.

Changes to the theme will affect all Projects using the theme, so if there are some projects that shouldn't receive the change then you are best to make a new Theme with the new setting.

You've noted you can see how to "edit heading level styles" - when you are in a text element there's an A/pencil icon in the mini toolbar that does open a Styles panel with an Edit option. If this is what you are referring to, it's important to know that these are Project styles, not Theme-level styles. They're actually inline styles that over ride the Theme settings, and are distinct from the Theme Types on the Format tab. So changes made to these Project styles don't change the settings for what's available in the Format tab >> Types.

You could also do inline styling to change an individual text element's font settings such as size. Here's an article on inline styling:

Working with inline text styling in Claro and Flow >> https://dki.io/bf407783

But this would only be helpful in selected cases, if you want to make an overall adjustment the Theme settings are the way to go.