  One of the biggest puzzles regarding the circumgalactic medium (CGM) is the
structure of its cool ($T\sim10^4$ K) gas phase. While the kinematics of quasar
absorption systems suggests the CGM is composed of a population of different
clouds, constraining the clouds' extent and spatial distribution has proven
challenging, both from the theoretical and observational points of view. In
this work we study the spatial structure of the $z\sim 1$ CGM with
unprecedented detail via resolved spectroscopy of giant gravitational arcs. We
put together a sample of Mg II$\lambda\lambda 2796,2803$ detections obtained
with VLT/MUSE in 91 spatially independent and contiguous sight-lines toward 3
arcs, each probing an isolated star-forming galaxy believed to be detected in
absorption. We constrain the coherence scale of this gas ($C_{\rm{length}}$),
which represents the spatial scale over which the Mg II equivalent width (EW)
remains constant, by comparing EW variations measured across all sight-lines
with empirical models. We find $1.4 <C_{\rm{length}}/\rm{kpc} <7.8$ (95%
confidence). This measurement, of unprecedented accuracy, represents the scale
over which the cool gas tends to cluster in separate structures. We argue that,
if $C_{\rm{length}}$ is a universal property of the CGM, it needs to be
reproduced by current and future theoretical models in order to understand the
exact role of this medium in galaxy evolution.
