This setting controls the maximum size of a message that can be sent by the producer. It also controls the cumulative size of all the messages in a single request. If set to 1MB, the producer can send a single message of size 1MB or 1000 messages of size 1K each. Note that the broker also has a setting that determines the maximum size of the message the broker will accept. A broker may still reject a request if the message size is larger than what it can accept.