Since IntelliJ IDEA is so much loved by keyboard addicts, here is a tiny yet helpful feature introduced in version 9.0.2: you can change size of a tool window by means of your keyboard.
Say, you’re doing “find usages”, the tool window is popping up and it’s too small for you to comfortably look at the usages tree. Now you don’t have to grab a mouse to stretch it up, just hit Ctrl-Shift-Up (Meta-Shift-Up for Mac users) and your tool window is getting bigger.
So, if you need to resize a tool window: focus it (by Alt-number or Ctrl-Tab) and then use Ctrl-Shift plus an arrow key that points to the direction you want.
You can also use these arrow shortcuts to resize dialogs and actually any window that IntelliJ IDEA shows. Say, you opened the project structure dialog and it’s too narrow for you. Use Ctrl-Shift-Right and Ctrl-Shift-Down to make it bigger.