  The outskirts of accreting dark matter haloes exhibit a sudden drop in
density delimiting their multi-stream region. Due to the dynamics of accretion,
the location of this physically motivated edge strongly correlates with the
halo growth rate. Using hydrodynamical zoom-in simulations of high-mass
clusters, we explore this definition in realistic simulations and find an
explicit connection between this feature in the dark matter and galaxy
profiles. We also show that the depth of the splashback feature correlates well
with the direction of filaments and, surprisingly, the orientation of the
brightest cluster galaxy. Our findings suggest that galaxy profiles and
weak-lensing masses can define an observationally viable mass-size scaling
relation for galaxy clusters, which can be used to extract cosmological
information.
