When you first begin using OneNote, new notes are formatted with the 11-point Calibri font. If you'd like to change the font, you'll have to go to the OneNote desktop app (OneNote 2016). For detailed steps for who to do this, see Change the default font of your pages.
Note: If you find the font in OneNote Online too small and you don't want to change it in the desktop app, you can use your browser’s zoom settings to temporarily change the size of what’s on the page. For example, in Internet Explorer, clicking the 100% magnifying glass button in the lower right corner of the browser window lets you control how small or large you want your notes to appear. In other browsers, look for a similar control.