The only way to change the font size in Airtable’s main interfaces is to use your browser’s “Zoom” functions, which are usually found in your browser’s View menu. This will make the entire page—including Airtable’s interface elements—bigger or smaller.

The only other ways to do it are to use apps, like the Page Designer, or various third-party solutions.

Is this a feature that could be added in future? It would be very very useful!