It is well established that small-scale structures have an important effect on the overall hydrodynamic behaviour of dense and circulating fluidized bed. Due to computational constraints, the numerical simulations of practical applications with Euler-Euler two-phase approach are usually performed with relatively coarse mesh with respect to the local segregation of solid. These simulations cancel out the small-scale solid structures. All previous studied attempted to take into account the effect of unresolved structures on the drag force in the case where the particulate phase is monodisperse. This paper is dedicated to analyse the unresolved structures effects on polydisperse gas-solid flow by multi-fluid Eulerian approach. In this study, the binary mixture is conducted by gas at an ambient condition in a 3D periodic circulating fluidized bed. The aim is first to obtain mesh independent results where further mesh refinement is not necessary. Then these results are used to investigate the unresolved structures effects on resolved field by following a priori methodology. In particular, the role of small-scale structures on the momentum transfer by inter-particle collisions is pointed out.