Hi Noel,
The sizes completely depend on how much your server available memory is and how big a data set user wants to see in run time.
I recently increased these parameter in our server for a similar issue and we have set it with
Max char stream size : 40 Mb
Max Binary stream Size : 150Mb
This configuration is working for us for fairly large data set however we had a large physical memory availability in our server (128 Gb)..