In this thesis I present a universal scheme to assess the macroscopicity of experimentally fabricated quantum superposition states based on the degree to which they probe the boundaries of quantum mechanics. The measure relies on the falsification of the weakest macrorealist modifications of quantum mechanics; it treats all experiments in an equal manner, regardless the actual form and size of the empirical data, using an uninformative approach to Bayesian inference.</br> The versatility of this scheme is illustrated by applying it on state-of-the-art quantum tests where the study of minimal modifications of quantum mechanics often coincides with the examination of conventional decoherence effects.</br> Finally, the environmental impact on orientational degrees of freedom, leading to diffusion, friction, and thermalization, are described analytically. Together with the formulation of minimal modifications of quantum mechanics applied to orientational degrees of freedom this paves the way for a thorough description of experiments with levitated nanorotors and the assessment of the respective macroscopicities.