Right. The size of a message depends on what particular message type you're looking at. You could try something like:

SELECT 
M.FullSize
FROM Ens.MessageHeader H
JOIN EnsLib_HL7.Message M
ON H.MessageBodyId=M.ID
Where H.MessageBodyClassName='EnsLib.HL7.Message'

but it depends on the implementation of the message type you're looking at.