Finally I could get my hands on an item I was searching for a long time (for an affordable price):
The german "Krieg der Sterne Super-Poster" from Kenner/General Mills/Parker, which you could pick up in german toy-stores in the good old days.
The seller told me, it was wallpapered on a wall of a WWII bunker in Hagen, which now is a kind of museum.
It's kinda worn up, but luckily they have endeavored to take it off the wall without destroying it.
This thing is huge with its 119 x 168 cm/ aprox. 47 x 66 inch.