How can I resize a Mac OS X app window after disconnecting from an external monitor?

Hold down the option key while clicking on the green button. This should prevent iTunes from changing to the mini-player, and will resize the window as expected.

For 99% of applications: The green button in the top left of any OS X window is called the "Zoom" button and will resize the window to it's contents so long as it fits in the current resolution. This is the same as choosing Zoom from the Window menu.

For programs such as iTunes that show a completely different mode when you choose Zoom, you can hold the option key when clicking the zoom button to force it to perform the normal zoom action instead of changing modes.

There's an alternative. Use some app such as BetterTouchTool that enables you to resize windows in different ways. On BTT I choose Fn + Option to resize window on top of the mouse.