  We address the macroscopic quantumness of the state of mechanical systems
subjected to conditional protocols devised for state engineering in cavity
optomechanics. We use a measure of macroscopicity based on phase-space methods.
We cover the domain from weak to strong optomechanical coupling, illustrating
how measurements performed over the cavity field that drives the dynamics of a
mechanical system are able to steer the latter towards large quantum coherent
states. The effect of losses is evaluated for the case of an open cavity, and
analyzed in terms of the features of the Wigner functions of the state of the
mechanical system. We also address the case of engineered phonon-subtracted
mechanical systems, in full open-system configuration, demonstrating the
existence of optimal working points for the sake of mesoscopic quantumness. Our
study is relevant for and applicable to a broad range of settings, from clamped
to levitated mechanical systems.
