Answer
See explanation.
Work Step by Step
Building a MOSFET requires making an insulating layer of silicon dioxide. If the device is too small and the layer is made too thin, a tiny amount of stray charge will provide a large electric field. It won’t matter that one of the p-n junctions is reverse-biased; current will flow freely even when the transistor is supposed to be “off”, defeating the purpose of the MOSFET.
Furthermore, if the MOSFETs are made too small, the carriers will tunnel through the insulating barriers that are intended to isolate adjacent transistors.
