IntelliJ how to zoom in / out

You need to look for the Increase Font Size and Decrease Font Size options on the Keymap menu, you can see the options on my screenshot. You will find the Keymap menu under Preferences > Keymap.

Assigning on those will have the expected effect for font zoom.

Double click Shift to open the quick actions. Then search for "Decrease Font Size" or "Increase Font Size" and hit Enter. To repeat the action you can doubleclick Shift and Enter

I prefer that way because it works even when you're using not your own Computer without opening settings. Also works without leaving fullscreen, which is useful if you are live coding.

Update: This answer is old. Intellij has since added actions to adjust font size. Check out Wilker's answer for assigning the new actions to keymaps.

Try Ctrl+Mouse Wheel which can be enabled under File > Settings... > Editor > General : Change font size (Zoom) with Ctrl+Mouse Wheel