The inbufsz property sets the size of the buffer on the input stream reading data coming in from a socket. Similarly, outbufsz sets the buffer size of the output stream used by the broker to write data to the socket.
In general, both parameters should be set to values that are slightly larger than the average packet being received or sent. A good rule of thumb is to set these property values to the size of the average packet plus 1 kilobyte (rounded to the nearest kilobyte). For example, if the broker is receiving packets with a body size of 1 kilobyte, the overall size of the packet (message body plus header plus properties) is about 1200 bytes; an inbufsz of 2 kilobytes (2048 bytes) gives reasonable performance. Increasing inbufsz or outbufsz greater than that size may improve performance slightly, but increases the memory needed for each connection.