Despite a century of active research on first-order phase transitions, discrepancies between predictions based on nucleation theory and experiments on nucleation rates are still of several orders of magnitude. This is partly due to the way the work needed to create a critical cluster is modeled. Here, using slightly modified classical nucleation theory, we reconsider confinement effect leading to one single and stable critical cluster. We relate the new cluster equilibrium size arising from confinement to the usual critical cluster size in infinite systems. The single and stable critical cluster opens new experimental horizons: it can be studied in detail. We stress the model-free nature of these results.