If your IP Thing publishes using the MQTT v3.1.1 specification, the payload size can theoretically reach up to 65,535 UTF-8 encoded characters. However, in practice, such large payloads are often impractical due to transmission delays and the risk of clients timing out before the message is fully delivered.
For reference, we successfully tested a 4 KB payload without any issues. If you have specific payload requirements, we’d be happy to work with you to ensure reliable and successful publishing.