Hello Stefan follow my tayp's and i HOPE it will work :
1. Right click on "CS 1.6 icon", go to properties and look in the target window. It should look like
"C:\installation Directory\Counter-Strike 1.6\hl.exe"
2. Outside of the quotation marks type
-width 800 -height 640
Now it should now look like this
"C:\Installation Directory\Counter-Strike 1.6\hl.exe" -width 800 -height 640
3. Now your CS starts in normal mode and you can access to whatever resolution you want.
NOTE: Finally remove "-width 800 -height 640" otherwise CS will start in 800x640 resolution everytime