Invoke Settings, then select GoLand | Settings for macOS or File | Settings for Windows and Linux. Alternatively, press ⌘ Cmd ,.

Then navigate to Editor > General > Change font size with Ctrl+Mouse Wheel. You can then increase or decrease the font size of your editor using the mouse wheel.

To increase the font size with your keyboard, use ⌃⇧. (macOS) / Alt+Shift+Period (Windows/Linux). To decrease it, press ⌃⇧, (macOS) / Alt+Shift+Comma (Windows/Linux).