How to select the value for spread constant while using RBF with nntool box ?

I am using nntool in Matlab for classification problem with Radial Basis function (Fewer neurons). Input = (7 x 500)matrix Target =(2 X 500)matrix. I am new to RBF.I request please help in choosing Spread function.I went through some of the threads available but I couldnot understand how to calculated Minimum distace.Please help me in understanding this ?