I'd (obviously? perhaps not) define the "size" of the simulation as the amount of information (i.e. uncompressible state) it contains, simply because for other

I'd (obviously? perhaps not) define the "size" of the simulation as the amount of information (i.e. uncompressible state) it contains, simply because for other measures of size many trivial simulations are infinitely large while having tiny incompressible information, so these other measures of size are not useful and not used. So a more compressible i.e. more regular, simpler simulation is the smaller one, and the simulation still has to be smaller than the simulator, and we literally don't care about the compressibility or incompressibility or uncompressed size, because why would we, that's not information-theory information.