Because larger transistors need more electrons, they also need more power to switch. I guess that circuitry made from valves/vacuum tubes might provide high radiation resistance?

I mean, you could just use big transistors

That's single event upset (SEU). The other radiation issue is total dose. That's when charge gradually builds up in insulating gates due to ionizing radiation. It's permanent, and eventually reaches the point that the transistor is always on so has a permanent fault. One of the factors is collection area, so larger gates are not less susceptible because they have a larger collection area. But if they run at a higher voltage that can make them more tolerant to total dose.

THe other problem is that it can bump the atoms permanently off the silicon lattice, so permanent damage. Bigger transistor can just survive more hits