> Does anyone know how large these would be compared to a civilian energy reactor?

Tiny. A civilian energy reactor has to implement two heat exchange systems -- one for transferring heat from the core, and one for heating water to steam to turn turbines and then condense it again.

For NTP there are no circular heat exchanges, and no turbines. It's just a hot core in your reaction chamber, which heats the hydrogen you squirt on it, and the hot hydrogen gas escapes out the rocket nozzle.

The smallest critical mass of plutonium is about four inches across. In theory that's all you need in the reaction chamber, but in practice you will also want cladding so that your hydrogen reaction mass erodes the cladding and not the plutonium (else you'll be squirting plutonium out the rocket nozzle along with your hydrogen), and a bisecting neutron reflector shutter or something so you can turn the core on and off.

So, maybe something about twelve inches across? Still much smaller than a civilian power reactor.