Beside changing the Maximum send size (for send connectors) and Maximum receive size (for receive connectors) in ECP (Exchange Admin Center).

In Exchange Management Shell, need to change Global Transport configuration settings.

To change Global Transport configuration settings for  MaxSendSize  to 30MB & MaxReceiveSize to 50MB:
Set-TransportConfig –MaxSendSize 30MB –MaxReceiveSize 50MB

To double check:
Get-TransportConfig | ft MaxSendSize, MaxReceiveSize
Get-SendConnector | ft name, MaxMessageSize
Get-ReceiveConnector | ft name, MaxMessageSize

Reference:
https://rajisubramanian.wordpress.com/2014/01/26/exchange-server-2013-message-size-configuration-detail/

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