The discharge behaviour of bulk solids is strongly influenced by outlet geometry. This study investigates the effect of outlet diameter on particle velocity at the outlet and on discharge rate during silo discharge. Airsoft balls and steel balls with a particle diameter of 6 mm were used as model bulk solids. The research combines analytical calculations, experimental measurements, and DEM simulations performed for outlet diameters of 100 mm, 150 mm, and 190 mm. Particle velocities were evaluated using the Protodjakonov and Daněk-Pavliska equations, while discharge rates were compared with the Beverloo equation and the British Materials Handling Board correlation. The results show that increasing outlet diameter leads to higher particle velocities and discharge rates. Good agreement between analytical predictions, DEM simulations and experimental measurements was obtained for the airsoft balls. For the steel balls, the comparison was limited to analytical and DEM predictions. The study confirms the applicability of the selected analytical approaches and demonstrates the usefulness of DEM simulations for investigating bulk solid discharge processes.