I'm not sure if this will work, but may be worth trying.
Go into System Preferences --> Displays. Select the Display tab so that the list of screen resolutions is visible. Do you have any listed below your current setting? If not, and you're already at the highest resolution, then my idea won't work.
But if you're not already at the highest resolution, choose one higher than your current one, but remember which one is your current one.
After changing the resolution, your iTunes window should be smaller to the point where you should be able to grab the lower right resize corner. Make it even smaller.
Now go back and change your screen resolution to what it was. Now resize your iTunes window to what you want.