This mostly probably only applies to people who have a Mac connected to an HDTV in their living room, or otherwise would be using iTunes visualizations as a nice effect at a social gathering; this is a different application of a similar hint. So, in light of that, here is the short version: iTunes full screen visualizer won't let you use the full width of a widescreen TV, or use high resolution. However,you can get the same effect by making the windowed visualizer huge, and then using Zoom from the Universal Access Panel.
Read the rest of the hint for more details, and some setup tips to make this process easier...
If you have ever tried putting the iTunes visualizer in full screen mode on a wide format display, you may have noticed that it either does not use the full width of the display (like watching standard programing on an HDTV). Even if it does, it is actually stretching a non wide-format image to fill the screen (most noticeable when the apple logo or track info are showing). You may have also noticed that the resolution itself switches to something fairly poor, 640x480 in most cases. Well, when you have a widescreen display, you generally prefer to use the whole display. But you probably do not want to just show the Mac desktop with a big window in it.
In this particular case, here is how you have your cake and eat it too: Make the iTunes window fill the screen, and run the visualizer in non-full screen mode. Then go to the Universal Access Panel, and turn on Zoom. Zoom the screen just enough so you only see the contents of the iTunes window. Presto, 100% visualizer effect.
Here are a couple of additional fine-tuning tips;
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050304101856347