Currently there are Small MuduleModule Reactors which use fission to generate an electricity output of less than 300 MWe. One of these, the NuScale produces 50,000 kilowatts/hour and is 76' by 15'.
https://www.hpschapters.org/florida/6spring.pdf
NASA is working on low-energy nuclear reaction(LENR) technology which they hope eventually to use to power cars, planes, and homes. https://www.extremetech.com/extreme/149090-nasas-cold-fusion-tech-could-put-a-nuclear-reactor-in-every-home-car-and-plane
MIT also claims to have made recent breakthroughs in an efficient fusion plant: http://news.mit.edu/2015/small-modular-efficient-fusion-plant-0810
Sadly the last two are theoretical. However as a reality check, it would not be unrealistic to expect nuclear reactors with a large output with the physical size of a home generator in the near future. And possibly the slightly further future may bring into reality AAA battery sized generators.
Human safety issues may be a concern, however NASAs LENR uses a nickel lattice and hydrogen ions and the reaction produces copper. Without more details this does not sound much more dangerous than the acids stored inside acid batteries.