Simply limited by the maximum travel of the Z axis, which is usually limited by the size of the frame. The maximum printing height can be increased by lengthening the Z axis on most printers. Some, such as the Prusa i2 or MendelMax, have the Y axis and Z axis lengths tied to each other, so it is not easy to scale one without scaling the other. Another consideration when scaling an axis up is the stiffness of that axis, including frame stiffness and linear guide (smooth rod) stiffness.
