Change Font Size in Notification Popups?

I’d like to reduce the size of the font used in the notification popups in 13.06. They’re a bit large for my tastes. Can’t find a setting for that. Is it doable?

This is not doable without modifying the notification code itself. It is inheriting the systemsettings.

Does it inherit from a particular entry in “Font Settings”?

Its not that easy. It inherits the default font of the current plasma theme. The current plasma theme then should get the font from the general font settings. (might be the desktop font settings also. I am not exactly sure)
Perhaps try changing the general and the desktop font and see which works for the notification popups.

To test the notifications you can create one like so

kdialog --passivepopup Test

when entered into a terminal.