For tokamaks, there's an idea that we can take all the relevant size and plasma parameters for a bunch of different experiments, run a fit on them, and come up

For tokamaks, there's an idea that we can take all the relevant size and plasma parameters for a bunch of different experiments, run a fit on them, and come up with a set of scaling factors that will tell us how well a reactor of size x is going to perform. The prediction is that the bigger the reactor, the better things get, which is why ITER is the biggest tokamak yet. People call it 'wind tunnel scaling', so I presume this is an empirical method borrowed from aerodynamic modeling. Nowadays we can actually do computational fluid dynamics to some extent, but a full-on electromagnetic plasma simulation remains intractable on the macroscopic/machine scale. And we remain limited on theoretical approaches, so we are stuck with actually having to build the darn things before we know they'll work.