  We propose a criterion which defines whether a superposition of two photonic
components is macroscopic. It is based on the ability to discriminate these
components with a particular class of "classical" detectors, namely a photon
number measurement with a resolution coarse-grained by noise. We show how our
criterion can be extended to a measure of the size of macroscopic
superpositions by quantifying the amount of noise that can be tolerated and
taking the distinctness of two Fock states differing by N photons as a
reference. After applying our measure to several well-known examples, we
demonstrate that the superpositions which meet our criterion are very sensitive
to phase fluctuations. This suggests that quantifying the macroscopicity of a
superposition state through the distinguishability of its components with
"classical" detectors is not only a natural measure but also explains why it is
difficult to observe superpositions at the macroscopic scale.
