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.
Enable Larger Fonts option in System Preferences > Display > Display > Scaled > choose an option to the left to the Default settings.
Note: It is available for MacBooks with a Retina display. For additional information and recommendations for non-Retina displays, please check out this page.
Or: Settings > Accessibility > Display & Text Size.
Read next: How Can I Change the Language in Spark?