It is recommended to use a horizontal axis. To be honest, vertical axis wind turbines are a lie in China. The only advantage is that they can capture wind energy when the wind direction is disordered. Others, such as low-speed generators, are nonsense.
For 500w fans, the blades of different manufacturers have different lengths.
The key now is that your motor is 500w. At what speed can the motor reach 500w? This is a necessary condition for selecting blades