The point is, that if you create a wavefunction for everything, then you can't do a measurement, because a measurement is external to the wavefunction.
It should still be impossible to tell "when" a kaon decays in this case.
I would argue that if you knew the exact quantum state of the components of the kaon, you could predict the decay. It appears random simply because of the random phase of the wavefunction of the undecayed particle. (The random phase assumption is of course a can of worms).
Likewise is there an absolute size of an object that can avoid de-coherence?
Decoherence of what with respect to what?.
I see what you're getting at now. By treating the entire universe as psi, there isn't any prepration or measurement available.
I'm not sure that ignorance of the phase is enough to cause the observed randomness (or that knowlege of the phase would allow prediction of decay paths.) There are some experiments for which one can determine phase. I don't remember the exact details.