There's probably an easier way but the way I worked around it on an ATI Card was to create a custom resolution on my smaller monitor that matched the larger monitor then set the smaller monitor's proper resolution to display. If you're going to try than do it at your own risk. It looks like you can set it here in NVIDIA:

- Open NVIDIA Control Panel
- Select Change Resolution
- Select the smaller Monitor
- Customize it to force the resolution that you want WoW to run at.
- Open Windows Screen Resolution by Right Clicking the Desktop and Select "Screen resolution".
- Adjust all Monitors to their appropriate Resolution and position.
- Set proper Resolution in WoW
