  One of the most fundamental characteristic of a complex system is its size
(or volume), which, in many modelling, is represented by the number of its
individual components. Complex systems under investigation nowadays are
typically large and/or time-varying, rendering their identification
challenging. We propose here an accurate and efficient method to determine the
size of (i.e., number of agents in) a complex, diffusively coupled dynamical
system, that leverages the response of the system to an injected probing
signal. For our method to be applicable, we rely on some assumptions on
system's characteristics, namely, on the spectrum of the coupling graph and on
the basin stability of its steady state(s). Even though such assumptions imply
that our method cannot be applied to any instance of diffusively coupled group
of dynamical agents, we argue that it covers relevant and interesting examples.
Furthermore, the simplicity of the approach and its low computational
complexity renders it very interesting for the systems to which it applies.
