In a previous work, we derived optimum conditions for the design of finite-size 1-D unidirectional leaky-wave antennas (LWAs) in the general case. The bandwidth and the gain-bandwidth figure of merit are, however, structure-specific and have not been discussed yet. A practical case that is widely investigated in the literature is that of 1-D unidirectional LWAs based on partially reflecting surfaces (PRSs). In this work, we not only evaluate these features for this specific class of antennas, but we also derive design rules for optimizing the PRS impedance and cavity height to get maximum gain. Numerical results and full-wave simulations are finally proposed for a realistic example based on a metal strip-grating structure, to validate the theoretical findings.