You can customize the font and font size used by a calendar object to display it's content by a large degree.
To change the font of a calendar:
Open the Adjust tab (Command-Shift-I) and select the calendar object in question.
Locate the Calendar - Year panel under the Adjust tab and click the Set Font button to open a font picker to change the font used for the year:
You will find the same Set Font button on the calendar panel for Months, Weekdays and Days.
To change the font sizes used by a calendar:
Open the Adjust tab (Command-Shift-I), select the calendar object in question and locate the Calendar panel.
You can set the Size attribute individually for the Year, Months and Day component of a calendar.
Please note, that a size value of 0 means, that the calendar will automatically calculate the font size proportionally to the size of the whole calendar object.
Choose the appropriate Size field on the part of the Calendar you like to change: