Length makes a difference on all rivers. 135m ships can't sail on the Seine, and on my recent Bordeaux cruise the captain told me that is not because of the locks on the Seine but because 135m ships can't navigate some of the bends. Most companies use 110m cruise ships on the Seine as a result, Viking have built custom length ships a little longer (124m if I remember correctly but annoyingly Viking always quote in feet).
I rejected MS Swiss Ruby for a Seine cruise, I'm not about to cruise the Elbe on her. The Seine gets many of the old ships in Europe, and some of them like Swiss Ruby are cramped with very small cabins. Eventually companies had enough 135m ships for the Rhine/Danube system that they started to move 110m ships to the Seine, at which point some of them were nice enough we did a Seine cruise with Riviera.