In all fairness modern naval reactors like the A1B are pushing more like 700 MW thermal (unclassified, probably higher) and float in what amounts to an infinite heatsink of excellent liquid coolant. Of course lifetimes and decades of experience produce the A1B, The First Torchship is not going to do as well.

The real problem for a spacecraft isn't generating 700 MW of thermal heat, which is pretty small and light, but radiating it away continuously. The ISS EATCS radiates about 70 KW and each radiator is in the thousands of sq ft and thousands of pounds. So four more digits would be tens of millions of sq ft and millions of pounds, VERY superficially. However the reactor probably doesn't have to be optimized for human temps, so maybe ten times to hundred times better? It would be quite large at any rate.

There are certain engineering optimizations you make if you have an infinite liquid heatsink like a naval reactor vs incredibly expensive cooling like a space reactor. If you're willing to boil sodium your condenser can radiate a lot more per sq ft than an ammonia based refrigerator for ISS HVAC. That's a little far fetched but the VHTR/HTGR design goal was a cool 1000 C, so the radiators can run quite a bit hotter and smaller than ISS HVAC systems.