You could probably get it crammed into something the size of a medium sized walk in closet or a very roomy bathroom for lower power models.
Speaking as an engineer, huge systems like nuclear reactors, chemical plants, etc are mostly piping and support systems like cooling or water treatment. Take that out of the equation and it gets a lot smaller.
Hell, wiki even mentions it.
"A water-cooled reactor is generally dwarfed by the cooling systems attached to it. Additional issues are that the core irradiates the water with neutrons causing the water and impurities dissolved in it to become radioactive and that the high-pressure piping in the primary side becomes embrittled and requires continual inspection and eventual replacement.
In contrast, a pebble-bed reactor is gas-cooled, sometimes at low pressures. The spaces between the pebbles form the “piping” in the core. Since there is no piping in the core and the coolant contains no hydrogen, embrittlement is not a failure concern. The preferred gas, helium, does not easily absorb neutrons or impurities. Therefore, compared to water, it is both more efficient and less likely to become radioactive."