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.
Currently, this is not an option for macOS devices. As a workaround let us share these steps: Go to System Preferences > Accessibility > Zoom > enable 'Use scroll gesture with modifier keys to zoom' > add CMD as a modifier key.
Or: Settings > Accessibility > Display & Text Size.
Read next: How Can I Change the Language in Spark?