I can't help you with any dimensions as I do not know the dimensions of the E34. But regardind to this installation guide for the electric sunblind, the rear shelves COULD be the same. If you already have the dimensions of the E34, I could measure my shelf in the 7-series, but in installed state. Getting the rear shelf out of the car is too much of a hastle...
http://www.e32-schrauber.de/bmw/date...elektrisch.pdf