Properties of tracers and voids in the Euclid Flagship mock catalog. Left: number density of spectroscopic galaxies and selected VIDE voids as function of redshift; vertical dashed lines indicate the redshift bins. Right: number density of the same VIDE voids as a function their effective radius R (void size function) from the entire redshift range, encompassing a total of N_v = 44 356 voids with 18.6 h⁻¹ Mpc < R < 84.8 h⁻¹ Mpc. Poisson statistics are assumed for the error bars. We refer to our companion paper for a detailed cosmological forecast based on the void size function (Contarini et al., in prep.).