  We consider fundamental limits on the detectable size of macroscopic quantum
superpositions. We argue that a full quantum mechanical treatment of system
plus measurement device is required, and that a (classical) reference frame for
phase or direction needs to be established to certify the quantum state. When
taking the size of such a classical reference frame into account, we show that
to reliably distinguish a quantum superposition state from an incoherent
mixture requires a measurement device that is quadratically bigger than the
superposition state. Whereas for moderate system sizes such as generated in
previous experiments this is not a stringent restriction, for macroscopic
superpositions of the size of a cat the required effort quickly becomes
intractable, requiring measurement devices of the size of the Earth. We
illustrate our results using macroscopic superposition states of photons,
spins, and position. Finally, we also show how this limitation can be
circumvented by dealing with superpositions in relative degrees of freedom.
