For anyone that is looking for the key in Windows 7 it is in one of the folders in:
HKLM\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\
Look for Position.cx
and Position.cy
in each of the subfolders; you have to edit the hex string.
Binary / decimal / hexadecimal converter:
http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html