@SGaist thank you, I took QFormLayout into account.
@VRonin also thank you!
I can not believe that it would take so much code to get this done...
After starting over and over again I came up with this:
and this looks exactly what I wanted, two widgets with a nice group box border sized to minimal space usage:
I dont really know If this code is correct or might produce trouble but at least I dont find any errors at the moment.