Shrunken screen size

After my mom connect this old laptop of mine to the LCD monitor, the size of the screen shrank.. leaving a big black frame on the outside regardless whether the laptop is connected to the monitor or not. After I wiped out the hard drive and reinstalled the operating system, the size of the screen remain shrunken...

I tried the old fashion Fn+(monitor switch) but it didn't help.

Ended up, the solution was simple... all that I need to do was to change the screen resolution to 1024 by 768 pixel from 800 by 600 pixel.....