If you want to zoom the iTunes video playback window to a given size, then simply ... control-click on the video area and choose a zoom level from the pop-up menu.
As the lower right window corner indicates, you can also drag the window to a different size.
[robg adds: This applies to iTunes videos playing back in their own window, not the window within the iTunes interface. A control-click there just allows you to set the poster frame for the video, as covered in a previous hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051122154335911