In the MPI Forum it says
The message envelope would normally be encoded by a fixed-length message header. However, the actual encoding is implementation dependent.
How can I find the size of the message header/ the message envelope for any given MPI implementation?
MPI_BSEND_OVERHEAD should be a good approximation of this size.
3.6.1 Model Implementation of Buffered Mode
The MPI constant MPI_BSEND_OVERHEAD provides an upper bound on the additional space consumed by the entry (e.g., for pointers or envelope information).
Except for bsend buffer allocation you don't need to know the size of the envelope.