The GPU driver should recognize the display/tv it is connected to, then provide you with a list of supported resolutions. So either the GPU driver isn't running (in which case you are limited to 800x600), or the display isn't getting picked up properly.
I'm guessing the second; I've had issues with GPUs picking up TVs properly, and I wouldn't be shocked if there's a setting you need to toggle to get yours to ID correctly.