We have theoretically studied the radiative decay of excitons in a film with nano-to-macro crossover thickness. Under the phase matching condition between exciton center-of-mass motion and the radiation field, the exciton radiative decay rate gets larger with increasing thickness because of the increment of the interaction volume. This is called exciton superradiance. However, it is suppressed at thickness over a particular length because the Fermi's golden rule is broken in exciton-photon interaction, i.e., the emitted photon begins to be reabsorbed and then the exciton behaves as a polariton. Its suppression condition is the main result of our study. We define an apparent propagation speed of the superradiant exciton as an effective thickness, which renormalizes the multiple reflection effect in the film, divided by the radiative decay time. This propagation speed also increases together with the thickness, and the exciton superradiance is suppressed when it reaches the group velocity of exciton polaritons. Afterward, the exciton radiative decay rate decreases inversely proportional to the thickness.