I agree, it's a titchy 'plane, but I think the KLM timetable (paper one; remember those?) did explain the maximum dimensions for hand baggage for each of the airlines in the KLM group. Should have been picked up at check in for the passenger's first sector, I suppose. I remember thinking that if the passenger had been flying to Heathrow rather than Stansted (or indeed to Stansted on a Fokker 100) he would not have had this problem.
P.S. If I recall correctly, the 'plane was in buzz colours.