We consider an ensemble of coupled nonlinear noisy oscillators demonstrating in the thermodynamic limit an Ising-type transition. In the ordered phase and for finite ensembles stochastic flips of the mean field are observed with the rate depending on the ensemble size. When a small periodic force acts on the ensemble, the linear response of the system has a maximum at a certain system size, similar to the stochastic resonance phenomenon. We demonstrate this effect of system size resonance for different types of noisy oscillators and for different ensembles -lattices with nearest neighbors coupling and globally coupled populations. The Ising model is also shown to demonstrate the system size resonance. DOI: 10.1103/PhysRevLett.88.050601 PACS numbers: 05.40.Ca, 05.45. -a, 05.50. +q Stochastic resonance has attracted much interest recently [1]. As was demonstrated in [2], a response of a noisy nonlinear system to a periodic forcing can exhibit a resonancelike dependence on the noise intensity. In other words, there exists a "resonant" noise intensity at which the response to a periodic force is maximally ordered. Stochastic resonance has been observed in numerous experiments [3]. Noteworthy, the order in a noise-driven system can have a maximum at a certain noise level even in the absence of periodic forcing, this phenomenon being called coherence resonance [4].Being first discussed in the context of a simple bistable model, stochastic resonance has been also studied in complex systems consisting of many elementary bistable cells [5]; moreover, the resonance may be enhanced due to coupling [6]. In this paper we discuss another type of resonance in such systems, namely, the system size resonance, when the dynamics is maximally ordered at a certain number of interacting subsystems. Contrary to previous reports of array-enhanced stochastic resonance (cf. also [7]), here we fix the noise strength, coupling, and other parameters; only the size of the ensemble changes.The basic model to be considered below is the ensemble of noise-driven bistable overdamped oscillators, governed by the Langevin equations,Here j i ͑t͒ is a Gaussian white noise with zero mean: ͗j i ͑t͒j j ͑t 0 ͒͘ d ij d͑t 2 t 0 ͒;´is the coupling constant; N is the number of elements in the ensemble, and f͑t͒ is a periodic force to be specified later. In the absence of periodic force, the model (1) has been extensively studied in the thermodynamic limit N !`. It demonstrates an Isingtype phase transition at´´c from the disordered state with vanishing mean field X N 21 P i x i to the "ferromagnetic" state with a nonzero mean field X 6X 0 [8].While in the thermodynamic limit the full description of the dynamics is possible, for finite system sizes we have mainly a qualitative picture: In the ordered phase the mean field X switches between the values 6X 0 and its average vanishes for all couplings. The rate of switchings depends on the system size and tends to zero as N !`.For us, the main importance is the fact that qualitatively the behavior of the mean...