If you can get the heat out, no reason it couldn't be tiny. A hayabusa motorcycle engine can be tortured into producing 1MW mechanical from about 5MW thermal.

If you can get the heat out, no reason it couldn't be tiny. A hayabusa motorcycle engine can be tortured into producing 1MW mechanical from about 5MW thermal. The heat engine required to meet efficiency would likely be the limit (although I think TEGs might be getting cheaper). The main issue is unless you have a big pile of them, you need 1% of your population to be nuclear engineers and security.