The critical mass of a bare sphere of plutonium-239 (the isotope usually used to make bombs) is about 10 kg. Some percent can be shaved off this with a neutron

The critical mass of a bare sphere of plutonium-239 (the isotope usually used to make bombs) is about 10 kg. Some percent can be shaved off this with a neutron reflector around the core and other design tricks. The American W54 warhead of the 1950s was 23 kg and the core was probably around one-third of the entire mass of the bomb. That is likely the practical limit for traditional designs. As for what might be feasible within the laws of physics, rather than current engineering, I'm not sure there's a clear answer. Increasing the density of the fissile matter reduces its critical mass, which is why they implode the core with conventional explosives in most designs. There are other ways of compressing matter to very high densities. For example, subjecting it to hot enough plasma, like in a magnetic confinement fusion reactor, might work. But stuff like that is science fiction and will be for a long time.