IANANE (I am not a nuclear engineer), but I don't think you can use water here, for the reason you state. Water makes sense on Earth because the temperature and pressure range suits it, but on Venus I think you'd need to find some other fluid to transfer heat to the outside, perhaps liquid sodium? But nuclear reactors work the same way as any other heat engine: you have a source of heat and exploit a temperature differential by moving heat from the heat-source to the environment. So it should work just fine on Venus, but with some other working fluid.

You're right though: this would be rather large. However, it might be possible to make it smaller, by eliminating shielding. Your smallest planned reactor surely uses a bunch of shielding, because it's intended for use on Earth, around humans. For an automated probe on Venus, that's not a concern so you don't need shielding, except whatever's necessary for safe handling before launch. This is also a place where it'd be a lot better if we had a Moon base where we could build things like this; it's a lot easier to launch mass from the Moon than from the Earth. Landing the probe should be easy: Venus's atmosphere is extremely dense so a parachute (resistant to the sulfuric acid, at least for a short time) should be fine, though the high windspeeds could be a problem.