I frequently use different size external monitors and sometimes (quite often) my iTunes window ends up the wrong size such that I can't drag the bottom-right handle.
I was about to write myself an AppleScript to resize the window to a fixed (small) size when I found this post, which answered the problem:
OPTION-CLICK on the green resize button. Done.
One thing I am beginning to get annoyed by with Apple is the undocumented (or hard-to-find) features that make things so much more usable!