How to select sigma size for Gaussian Filter ??

We have Gaussian filter defined as G(x,y)=e^(-(x^2+y^2)/(2*sigma^2)) according to Gonzalez.

Can somebody please tell me how to choose the value of sigma???

Also please tell me how to choose the value of n, if n*n is filter size?

Acc. to Gonzalez n>=6*sigma and n is odd. So if sigma=4. n=25. But using these values gives a White image (Blank Image)

For me sigma=.2 and n=25 is working. Any comments???