The default font size for Lessons can be adjusted on the Lessons Tab:
Select Save to store your changes
When you preview a lesson you'll notice there are two font size options. Changing the base font size to a larger number will make all lessons in your workspace appear with bigger fonts. If you change the Bigger font size option, selecting the large A (in the image below) while in a Lesson, will reveal the effects of this change.
Note: You can also revert back to the Coassemble default fond sizes by selecting Restore Defaults