You can set a global font face and size for all events in your calendar in BusyCal > Preferences > General.
You can apply custom fonts and styles to selected events using the Font menu.
You can apply font styles to notes in the info panel or the List View notes pane. In the List View, the notes pane shows a ruler with additional formatting controls.
If you'd prefer not to display rich text in your calendar, you can toggle the display of styled text by selecting View > Show Rich Text.