While viewing or editing notes in OneNote Online, you can use your browser’s zoom settings to temporarily change the size of what’s on the page without messing up its formatting. 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 to do the same.
Zooming is also available in the OneNote desktop app. For info about how to do this in OneNote 2016, see Zoom a notes page.