  This work discusses the optimal reconfigurable intelligent surface placement
in highly-directional millimeter wave links. In particular, we present a novel
system model that takes into account the relationship between the transmission
beam footprint at the RIS plane and the RIS size. Subsequently, based on the
model we derive the end-to-end expression of the received signal power and,
furthermore, provide approximate closed-form expressions in the case that the
RIS size is either much smaller or at least equal to the transmission beam
footprint. Moreover, building upon the expressions, we derive the optimal RIS
placement that maximizes the end-to-end signal-to-noise ratio. Finally, we
substantiate the analytical findings by means of simulations, which reveal
important trends regarding the optimal RIS placement according to the system
parameters.
