Cosmological forecasts for the Euclid mission from the void size function for the wCDM model, characterised by a DE component described by a constant w. The contours represent the 68% and 95% confidence levels obtained by means of the Bayesian statistical analysis described in Sect. 3.3. Left: forecasts for a cosmological model with w and Ω_m as free cosmological parameters. We report the constraints obtained by fixing the calibration parameters with blue contours marked by a solid line and the results obtained by relaxing the calibration constraints with light-blue contours marked by a dashed line (see Sect. 3.4). Right: forecasts for a cosmological model with w and M_ν as free cosmological parameters. We represent the results of the fixed calibration case as red confidence contours having solid borders and those of the relaxed calibration case as orange contours having dashed borders. For each plot we show also the constraints on σ₈, computed as a derived parameter. The true values of the parameters are shown by a black dashed line.