Believe it or not the diameter of a marble can be determined without ever directly measuring it.
The method is similar to how physicists use a particle accelerator to fire particles at an object they wish to investigate. By repeatedly rolling marbles at a target marble that is sitting within a measured area the diameter of the marbles can be calculated using probability. 100 marbles of equal size are provided so that you can have multiple groups conduct this experiment. Prove that it can be done you may be surprised at the accuracy of measuring an object solely through collisions.