This feature is in preview currently and is enabled for premium namespaces.

• XML Property - "MaxMessageSizeInKilobytes" (add it after all other properties)
• Add new API version - "2021-05"
• Add new get/set property MaxMessageSizeInKilobytes to QueueProperties, TopicProperties, CreateQueueOptions, and CreateTopicOptions
• No validation on the client - let the service handle validation
• Docs - Gets or sets the maximum message size in kilobytes for messages sent to this queue/topic. This feature is only available when using a Premium namespace and service version "2021-05" or higher.