Molecular dynamics simulations have been employed for the study of photolysis
of hydrogen bromide placed inside or on the surface of Ar$_{12}$, Ar$_{54}$, Ar$_{97}$,
and Ar$_{146}$ clusters, representing one to three icosahedral solvation shells. A large set
of classical Wigner trajectories, which take into account the initial quantum
rotational or librational delocalization of HBr,
is generated and analyzed in terms of transient hydrogen populations inside the
cluster and final kinetic energy distributions. 
The key result is that for fully solvated
HBr the size effect on caging is dramatic in the studied range of cluster sizes, while it is only
moderate for surface isomers. Simulations also demonstrate that caging can be efficiently
'turned off' by a librational preexcitation of HBr on argon clusters.
Calculations are compared to results
of cluster experiments, which have measured the kinetic energy distribution of hydrogens
originating from HBr photolysis at 243 nm in or on argon clusters with an average size of
115 atoms, and a near-quantitative agreement is found.