The "Socket buffer size" option controls the value of the SO_SNDBUF and SO_RCVBUF socket options set using setsockopt
The "Internal transfer buffer size" is the size of the buffer FZS uses internally. It's basically the maximum amount of bytes FZS tries to handle in a single IO operation.
In both cases, if the values are too small, FZS will be more busy to process the event loop than to do the actual IO, leading to poor throughput.
If the values are too big, lots of CPU resources will be wasted in moving stuff around in the buffers (memcpy and such).