Microwave energy receivers - what limits how small one can be?

[SomeGuy12]

So I was thinking about the optimal "lander" for exploring the various vacuum moons in our solar system. Optimally, you'd have a mothership with a powerful nuclear-electric power generator of some type. Best way seems to be aneutronic fusion where you decelerate helium ions escaping from the reaction for direct power conversion, but if that never ends up being workable, you could use a conventional nuclear reactor to pump hot sodium in a circle and extract energy from the moving sodium directly.

Problem is, the reactor is heavy, and then there's all those heat radiators and the radiation shield. There's also no convenient place to put the reactor on a lander - you have to have this big heavy radiation shield as well, and you want part of the shielding to be distance, so the reactor should be on a long boom away from the astronauts. Just doesn't work for a lander design though.

So your lander has a "roof" of a microwave receiver array. You have a larger array on the mothership beaming power to it. You use some kind of efficient but also lightweight electric thruster. I read that MPDs are a good way to do it, because the thruster itself is very lightweight and simple, and you only need it to work for about 10 minutes on landing and ascent and then you can rebuild it in the machine shops on the mother-ship. Maybe a few hours of total burn time if you are using the shuttle to ferry materials to and from the surface.

You use hydrogen as propellant.