Classifying hand written digits dataset using Probabilistic neural network (PNN)?
Reducing the sample size is pretty much up to you.  PNN have the drawback that the size of the network after training is equal to the size of the training dataset.  Therefore it's a bit impractical to use for large datasets.  I also believe that there is evidence that suggest the performance may also decrease when the layers are big.

I don't have access to the paper but I imagine a big reason why they reduced the sample was to avoid the memory error that you're encountering right now.  I also suspect they carefully selected a good representation for the sample.