The current size is the limits for the larger tumbler. The tumbler is donut shaped, so there's a "hole" in the middle where you can't tumble parts. Also, it's not like printing where you can print along the edges, there needs to be a certain amount of space for the polishing beads to vibrate and flow. Adding those factors up and you have a bounding box limit of 15x15x15 cm.
Right now, we are not planning to on board a bigger tumbler machine unfortunately. The next thing we want to experiment with is coloring + tumbling.