I think I read somewhere that Eric said making the focus-fusion device smaller was technically better, but that the electrodes needed to be a certain minimum size so they are not damaged by the electro-magnetic forces on them during pinch, which limits shrinking the device. Someone more informed please correct me if I’m wrong.
Also, the focus-fusion device actually IS pretty small. The reactor is about the size of a coffee can, if I am not mistaken. I am not sure what the size of the X-Ray capturing “onion” around it is projected to be. I think a big part of a real device is the shielding required by the radiation from the side reactions, which might not be reduced significantly for a lower power reactor because the side reaction radiation would still be the same type and energy, just with a lower count.
I expect that the geometry of a focus fusion device and the temperature and pressure requirements for fusion ignition have a significant influence on the voltage and current required for pinch, no matter how much reactant is desired to be fused. The pinch voltage and current determine the input power. If this is the case and only a small amount of reactant were fused, the device would not achieve break even. To achieve a lower output one could run the device at a lower pulse rate, which would reduce the maintenance requirements, but I do not see how that would make the device significantly smaller.
Other technologies than focus fusion would scale differently, but in general fusion requires a high enough temperature and density for ignition. For technologies I am aware of this requires significant energy, so they will not achieve break even without enough reactant. However, this argument does not apply to non-pulsed technologies such as Tokamak, which have their own problems if the device is too small.