Thanks all for your replies.
I’m new with the UV mapping too, I dont actually know what it is, but I see thats the scheme with the boxes and lines. Maybe like an input/output system? I always thought you were talking about ultra violet something.
But once I set up the correct scale, does it stay the same if I drag and resize the wall afterwards? Or do I have to set the scale always last? So I have to calculate everytime how much I have dragged a wall?