Here is another compact marble machine with motorized movement that puts on a show on your desk. The 3D Printed Marble Run is USB powered. It has a footprint of 6 x 6 x 2 inches. According to the maker, Paul Kemp, this machine accepts 8mm marbles. The 30 marbles supplied move 5.4 miles in an hour in this machine.
This marble toy is currently being sold on Etsy. You can find out more about it here.