Method 1 - Using script mediatoryou can use the scrip mediator to find the size of the complete message. Following is example how you can do it,
In this sample it gets the string length of message, and assign the value to a property. And then read the value from outside the script mediator and log it.
Also you can only get the length of the payload of the message by calling mc.getPayloadXML() inside script mediator.
Refer  for more information on script mediator.
Method 2 - Read the Content-Length headerNote that this method can be only used if the header is since it's not a required header. Value of the Content-Length header can be read as follows,
Please reply if you have any alternative or better methods other than these to find the SOAP message size inside a proxy service.