Gas cyclone separators are employed to separate particles from gas. In this study, the effect of the vortex finder diameter, inlet velocity and particle size on the flow field and the performance of a large industrial Stairmand cyclone has been studied both experimentally and numerically. The vortex finder diameters used are 0.40, 0.45, 0.50 and 0.55 times the cyclone diameter. Cyclones with body diameters of 700mm and 254mm are used. Cyclone inlet velocity is changed from 11.5m.s(-1) to 19m.s(-1). Particle size is varied between 1 and 13 mu m. Whether the correlations obtained for small-scale cyclones are valid for large-scale cyclones has been investigated. The three-dimensional numerical study is carried out by using ANSYS Fluent 17.0 software package for incompressible turbulent flow condition. Reynolds stress model is chosen as the turbulence model. Sawdust ash is used as particles. The results of numerical study are compared with the results of experimental study and literature. Results are found to be consistent with each other. It is seen that cyclone collection efficiency and pressure drop increase when both vortex finder diameter decreases and inlet velocity increases, but 50% cut-off diameter decreases. Results show that the correlations obtained for small sampling cyclones may not be appropriate for large-scale cyclones.