There actually is no limit set for message size in API-M. However, you should keep in mind that the latency of the API would increase with the increase of the message size. This is highly dependent on hardware resources and other relevant factors. You can refer to this graph[1] to get an idea of the impact of message size on response time. Therefore for a limit to be recommended, a load test needs to be done on your current API-M implementation to determine a maximum size based on your allowed maximum response time.

If required, you could also define a maximum message size by uncommenting the following configuration in <APIM_HOME>/repository/conf/passthru-http.properties and changing the valid.max.message.size.in.bytes.

message.size.validation.enabled=true
valid.max.message.size.in.bytes=81920

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