You probably already know that you can view album art (or a movie or other video) in a separate window in iTunes by just clicking on the image in the Now Playing box.
But did you know you can then control-click on the image in the separate window for some extra viewing options? Control-click and choose your preferred viewing size for the artwork or video from the pop-up menu.
[robg adds: These options are also available in the View menu, or via keyboard shortcuts. I wasn't, however, aware of the contextual menu.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061001204243511