VMS Help Sys Parameters, DEFMBXMXMSG  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    DEFMBXMXMSG sets the default for the mailbox maximum message size
    in bytes when this value is not specified in a Create Mailbox
    ($CREMBX) system service call.

    DEFMBXMXMSG is a DYNAMIC parameter.