The C1DatePicker inherits from the C1Button and exposes a C1MonthCalendar (Calendar property).
You can set the font properties on the main control as usual:
c1DatePicker.TextBlock.FontSize = xxx;
Another option would be to change the C1ControlBase.DefaultFontSize property before creating the control:
C1ControlBase.DefaultFontSize = 10;
c1DatePicker = new C1DatePicker();
I hope this helps.