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I believe the limit to scale is in the structural integrity of the medium. Increasing rotational speed stresses the crayon wax, and it could fling apart. Using heat periodically to allow the structure to cool and solidify would be important, especially when adding greater rates and higher temperatures of molten mass, as would be necessary for the material to make it to the end of the longer stalactites, and it would heat and weaken the structure without periodic cooling. I think achieving wheels on the scale of a car wheel is highly probable.