Take a look at the Overscan/Underscan options in the GPU's control panel, it might also be forcing the wrong aspect ratio on the display / carrying it over from the laptop's default. There should be an entire section in the control panel for desktop sizing and setting screen borders for additional displays.
The default for those laptops is an Intel embedded GPU, take a look at `Scaling` and possibly `Full Screen` option if it's there under General Settings tab. Scaling should let you increase the height and remove the borders. Play around with the options in `Custom Resolutions` this is where you'll find the Underscan changing ability.
