I don't think that the size of a diorama matters that much, I think its more the old sets that kids never knew about that draws the attention.
Geobra's setups are limited to the latest batch after all, its meant to boost sales. Making a diorama of stuff that cant be found in stores anymore wouldnt help that much for that.
And huge diorama's would be very much out of reach for kids. fun to see, but unless they have parents who are playmobil collectors, it wont be anything they will ever have at their house.
A diorama consisting out of pretty much the full batch of a theme however would be within reach.
Personly, I'd advice Geobra to use the middle ground here: Let a collector set up a diorama using only items currently in stores or in DS.
From a marketing point of view, it would be the most effective way to boost sales I bet.