Changing the font size on your calendar is easy!
FROM THE WEB APP
Step 1: Go to Calendars
Step 2: Click the settings 'gear' icon in the top right hand corner.
Step 3: Select 'Calendar Event Font Size'
Step 4: Done!
FROM THE MOBILE APP
This setting is only available on the web app. The font size for the mobile app is controlled by the font size settings for your phone.