I'm training the self organizig map, I need to set the value for the radius of it. is there any method to find the optimum radius size ?
It depends on the range and scale of your input data. If you are mean-zero standardizing your feature values, then try $\sigma=4$. If you are normalizing feature values to a range of [0,1] then you can still try $\sigma=4$, but a value of $\sigma=1$ might be better. Remember, you have to decrease the learning rate $\alpha$ and the size of the neighborhood function with increasing iterations, as none of the metrics stay constant throughout the iterations in SOM.
It also depends on how large your SOM is. If it's a 10 by 10, then use for example $\sigma=5$. Otherwise, if its a 100 by 100 map, use $\sigma=50$.
In unsupervised classification, $\sigma$ is sometimes based on the Euclidean distance between the centroids of the first and and second closest clusters.