  Optomechanics is currently believed to provide a promising route towards the
achievement of genuine quantum effects at the large, massive-system scale. By
using a recently proposed figure of merit that is well suited to address
continuous-variable systems, in this paper we analyze the requirements needed
for the state of a mechanical mode (embodied by an end-cavity cantilever or a
membrane placed within an optical cavity) to be qualified as macroscopic. We
show that, according to the phase space-based criterion that we have chosen for
our quantitative analysis, the state achieved through strong single-photon
radiation-pressure coupling to a quantized field of light and conditioned by
measurements operated on the latter might be interpreted as macroscopically
quantum. In general, though, genuine macroscopic quantum superpositions appear
to be possible only under quite demanding experimental conditions.
