You should be able to make a reactor with anything larger than a couple kg of material.

Maybe you can avoid a thorium reactor if that size from being a valuable target. I have little idea about the social risks of somebody using the material as poison, instead of explosive, but I suspect it's incredibly overblown currently. (Wannabe thieves are probably way more effective going after something else.)

But well, don't expect very small reactors to be efficient or cheap. They maybe could be cheap heat sources, but electricity generation and distribution are full of scale efficiencies.