By default, new notes use the 11-point Calibri font in OneNote, but you can change the font and the font size for the new notes that you create.
On the menu bar, click OneNote > Preferences.
In the OneNote Preferences dialog box, click Edit & View, and select the Font and Size you want OneNote to use.
Your new default font choice applies to new notes you write. Existing notes won’t be reformatted automatically.
Changing the default font size applies to the note text only, not the page titles.
More related info
If you’re interested in formatting only some sections of the text, and you’re not changing the default font for all of your new notes, select the text you want to change and click the Home tab. You’ll find text formatting options, such as font size, color, and style, on the ribbon.