Hi,
Are you sampling the data over certain time frame?
Rayleigh approximation is generally used to approximate channel fading where there are too many objects and there is no line of sight between transmitter and receiver. So, you may sample the data over 1 sec of time period. I think you can give any number of large matrix. More data, better you will get approximation. If you have some reference paper or some more detailed statements please provide me. I am happy to help you out more and have more detail discussion.