The IntelliJ keymap says:
Zoom in: Keypad + =
Zoom out Keypad - -
But they have no effect. Anyone have this working?
New info: Now I have added more key-bindings:
Zoom in: Alt-Shift-=
Zoom out: Alt-Shift-minus
But these still have no effect. Does zoom in/zoom out even work at all in IntelliJ?
This question is related to
intellij-idea
zooming
The answer is
Before User Shift + = or Shift - , you have to first set the key map as mentioned below 
I assume that you have a similar view regarding the zoom functionality as I have in this picture:
Now if you mark one of the Zoom In/Zoom Out lines and choose Add Keyboard Shortcut:
You will find that this particular shortcut Numpad + is already occupied so there is a conflict:
So you'll just have assign this Zoom In/Zoom Out to some other keyboard shortcut:
Update for intellij idea 2017 3.2
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, type zoom and switch zoom to on
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
File>Settings...>Editor>General>
then you'll find this in the menu Mouse,
"change the font size(Zoom) with Ctrl+Mouse Wheel"
Source: Stackoverflow.com