It isn't affected by that since the launcher is hidden. However, on a narrow screen (screen is tilted, so it becomes 1080x1920) it becomes needlessly long, just as it would be if it were flat in a wide screen setup.
But there are many other reasons to use Unity in any case. The keyboard support is awesome, for instance. So the answer to your question is that Unity is very useful on any screen that is at least 800x800. Smaller than that would be more difficult, I think, but that has more to do with Gnome than Unity.
You can add other panels and stuff if you like to, by the way. For instance, if you want a taskbar like Gnome 2 and Windows XP, then you can install http://apt.ubuntu.com/p/xfce4-panel. Afterwards, press alt+f2 and run xfce4-panel
to start it. Obviously, if you wanted to use it, then you'd add it to startup applications. The point is that you can combine Unity with other things, like awn or ...