Solved Screen shrinked after testing 2 screens on my laptop Sparky-Mate-desktop

[berliner58]

Hello,
sorry for my bad english, I am german.

I have Sparky Linux Mate-desktop  on my laptop Toshiba-Satellite uefi-64-bit.
Since my PC and the Dell-monitor don't work when connected with VGA but only with DVI connection,
I tried to test if  VGA will work when I connect the Dell-Monitor with my laptop.

Sparky on my laptop recognized the Dell-monitor and I selected the option to mirror both monitors.
Unfortunately the Dell-monitor remained black, maybe the VGA-thing is bad and doesn't work anymore.

But now my Sparky-laptop-screen has shrinked to the size of the Dell-screen.
It shows black screen on the left and on the right side of the visible screen. Looks like 4:3 TV on a 16:9 monitor where black stripes on the left and right of the movie-screen are seen.
In the screen-settings there is nothing I can do to expand the visible screen to the full size, it stays shrinked.

I assume there is a solution, but have no idea.
All the debian-forums I searched in for a solution only discussed resolutions but not a shrinked screen.

I hope you understand what I am trying to express ?

Edit:  I found the solution here
http://sparkylinux.org/wiki/doku.php/monitor_resolution