Two different settings affect the display size of the virtual device. The actual size of the hardware screen of the particular emulated equipment (Nexus One, Nexus S, ...) and the scale factor used by the emulator when rendering this display on the screen of your computer. Both are properties of an Android virtual device, so reinstalling Eclipse will not change them, but you can edit them in the Android Virtual Device Manager or when you start your emulator.
If you run the emulator from the command line, the third paragraph of the command line help page describes how to change both settings.
If you run the emulator from Eclipse, you can see the launch configuration that you use to debug your application. At the very bottom, you can give the scaling factor and other properties as additional command line arguments. Make sure it is not installed.
If nothing still works, just create a new virtual device for your emulator in the Eclipse toolbar.
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