Most commercial nuclear fission reactors are large, on the 1000MW order of magnitude. Some of the early experimental reactors were small, and the reactors in nuclear submarines are relatively small. I understand that the Navy even has one very small (crew of 7 people, 400 tons v. 7,000-9,000 for nuclear attack submarines) nuclear submarine that also moves very slowly 4 knots, which does searchs of the ocean floor (e.g. for wreckage). See here: http://www.chinfo.navy.mil/navpalib/factfile/ships/ship-nr1.html A couple of commercial ships of a freighter size were once nuclear, but have been converted back to conventional fuel. I've also heard about self-contained small nuclear reactors that they're talking about trying in an Alaskan village (on a pebble bed concept, IIRC). See http://www.adn.com/front/story/4214182p-4226215c.html What are the practical limits that impact the scale of a nuclear fission reactor? For example, what prevents someone from developing a nuclear powered airplane?