In the book Early Riser by Jasper Fforde, humans hibernate in large dormitories. These circular buildings range in size from 20 to 60 floors, with around two dozen rooms per floor. While the residents slumber they are kept nice and warm.
The heat is generated by a hot pot. The hot pot is a small nuclear reactor. A lone steward stays awake during the winter operating the hot pot by partially inserting and removing control rods to regulate the temperature. The reactor is set up to produce heat, not electricity.
This seems like a good way to heat a very large building. Even small nuclear reactors have to be quite large and expensive to function properly and safely. The buildings residents are almost always asleep while the building is occupied, so there is almost no energy usage except for heat. What is the smallest building size that would match with the energy output of a small nuclear reactor?