Going much smaller than we have right now, and you run up against some exotic issues--if you decrease the dielectric thickness, you run the risk of break down between the gate and source/drain under normal operating conditions. Your steady current rating also drops dramatically if the other feature sizes are decreased. I would probably say a 3 atom MOSFET is not possible, since at that scale you can't really have an effective dielectric between the atoms, and the conductors connected to the "single-atom terminals" will have a bulk effect on the behavior.