The PROPID_M_BODY_SIZE property indicates the actual size of the message body.
PROPID_M_BODY_SIZE is only used when reading messages from a queue. To find the size of a message, pass PROPID_M_BODY_SIZE to MQReceiveMessage and look at the returned value. PROPID_M_BODY_SIZE returns the actual size of the message body.
When passing PROPID_M_BODY_SIZE to MQReceiveMessage, the corresponding VT field in the aPropVar array can be set to VT_NULL.
MQReceiveMessage, PROPID_M_BODY