  Recent experiments claiming formation of quantum superposition states in near
macroscopic sys- tems raise the question of how the sizes of general quantum
superposition states in an interacting system are to be quantified. We propose
here a measure of size for such superposition states that is based on what
measurements can be performed to probe and distinguish the different branches
of the state. The measure allows comparison of the effective size for
superposition states in very different physical systems. It can be applied to a
very general class of superposition states and reproduces known results for
near-ideal cases. Comparison with a prior measure based on analy- sis of
coherence between branches indicates that significantly smaller effective
superposition sizes result from our measurement-based measure. Application to a
system of interacting bosons in a double-well trapping potential shows that the
effective superposition size is strongly dependent on the relative magnitude of
the barrier height and interparticle interaction.
