You can change the font size of Spark’s UI to either make the app font smaller so more content can fit on the screen or set a larger font to make emails easier to read. Spark supports Dynamic Type and will fetch the settings and adjust automatically.
Change the font size
Enable the Larger Fonts option in System Settings > Displays > Larger Text.
Note: It is available for MacBooks with a Retina display. For additional information and recommendations for non-Retina displays, please check out this page.