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You can keymap the usual Cmd +/-/0 in the IntelliJ (or Android Studio) keymap settings by searching for font size in the keymap. Go ahead and set it as shown in the screenshot, and IntelliJ will ask you whether you want to remove it from other assignments. This works well and it should be the default!
For Pycharm, it is in settings->editer->General->Mouse->Change font size (Zoom) with Ctrl+Mouse Wheel.
In the editor, press Alt+Shift+. to increase the font size globally or press Alt+Shift+Comma to decrease it. CLion displays a zoom indicator on the bottom of the editor that shows the current font size and the default one to which you can reset the current font size.
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.
Try Ctrl+Mouse Wheel which can be enabled under File > Settings... > Editor > General : Change font size (Zoom) with Ctrl+Mouse Wheel
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 for intellij idea 2017 3.2
Before User Shift + = or Shift - , you have to first set the key map as mentioned below 
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