The cross-flow, or vertical axis tidal turbine, is a prominent configuration of marine renewable energy device aimed at converting tidal currents into electrical energy. This paper highlights the hydrodynamic limitations of laboratory testing such devices and uses numerical simulation to explore the effect of device scaling. Using a 2D Reynolds-Averaged Navier–Stokes (RANS) numerical approach, a single turbine blade is initially modelled and validated against published data. The resultant numerical model is then expanded to emulate an experimental cross-flow tidal turbine designed and tested by the University of Oxford. The simulated turbine achieves a close quantitative match for coefficients of power, torque and thrust, forming the basis of a study exploring the effects of Reynolds number scaling in three alternative operating conditions. It is discovered that the coefficient of power (CP) increases with Re without a ubiquitous correlation until an Re of ∼350,000. Above this Re the CP values for all three operation conditions become both proportional and predictable. The study represents a significant contribution to understanding the application of detailed numerical modelling techniques to cross-flow tidal turbines. The findings, with regard to scaling from laboratory data, could reduce uncertainty and development costs for new and existing devices.