Highly correlated photon sources can be realized through cooperative coupling among quantum systems, giving rise to superradiant collective emission. In solid-state ensembles, however, such collective behaviour is typically confined to subwavelength dimensions (the Dicke limit) and strongly suppressed at room temperature by inhomogeneous broadening and rapid dephasing, hindering practical implementations. Here, we show that molecular J-aggregates sustain room temperature superradiant emission and enter a highly collective regime when strongly coupled to the delocalized photonic modes of a silicon bound-state-in-the-continuum (BIC) metasurface. J-exciton polaritons exhibit markedly enhanced, excitation-density-dependent cooperativity, evidenced by an increase in the zero-delay second-order correlation to g^((2))(0)=3.77, well within the superbunched emission regime (g^((2))(0)>2), and by the growth and reorganization of spatially synchronized emissive domains over areas as large as 8.5 um^2, far exceeding the characteristic Dicke area (λ^2) at the emission wavelength of 589 nm. An interacting-emitter model identifies superradiant clustering as the mechanism by which metasurface-mediated interactions extend J-exciton cooperativity, establishing resonant dielectric metasurfaces as a route towards micrometre-scale superradiance and enhanced photon correlations in disordered materials.