Those particle energies come from D-T fusion. Helion is doing a hybrid of D-D fusion, which produces lower-energy neutrons comparable to fission, and D-He3 fusi

Those particle energies come from D-T fusion. Helion is doing a hybrid of D-D fusion, which produces lower-energy neutrons comparable to fission, and D-He3 fusion which does not produce neutrons. They say the combined reaction will produce only 6% of its energy as neutron radiation, compared to 80% for D-T. There will be some D-T side reactions, but not much. Reactor size depends on the design. For tokamaks, output scales well with reactor volume. But it also scales with the fourth power of magnetic field strength. Two well-funded startups are building tokamaks with newer superconductors that support stronger magnetic fields, allowing them to get the same output as ITER from much smaller reactors. Other designs have different scaling laws so aren't necessarily any particular size. Helion for example is pretty compact.