Each monitor will have its own aspect ratio and maximum resolution. in the Video settings
Each graphics card will have a maximum resolution too.
Right click your desktop > properties > settings, there will be a slider for resolution, change that until you are comfy. However if the monitor goes black, just don't press anything and wait 15 seconds and it will revert to the last used resolution.
4:3 - TFT
16:9 - Widescreen
16:10 - Widescreen
12:3 - Triplehead