If you can't select a resolution larger than 800x600 or 1024x768, your system is likely in Clone mode. In Clone mode, the resolution is set to the largest resolution that all displays support. To select a separate resolution for each monitor, you must switch to Extended Desktop mode:
- Right-click on the Desktop and select Graphics Properties.
- Click Display, select Multiple Displays.
- Select Extended Desktop mode and click apply.
- You can now select the largest resolutions for each monitor.