Hi Daniel.
There’s no setting for making the widget smaller. The widget is dynamically sized to its container. For example, if you have a 20% width left sidebar and an 80% width content area and put the Mini Calendar widget in the 20%, it’ll take up that much space.
If you put it in a 10% width left sidebar, it’d be a more compact display. However, I don’t believe the font-size would decrease.
Beyond this, you’d need to customize your site’s CSS to target the Mini Calendar widget.
If you need some coding help, you may want to ask your developer or reference our documentation and list of known customizers.
Please let me know if you have any follow-up questions on this topic.