/sys$common/syshlp/helplib.hlb BACKUP Command, Qualifiers, /BLOCK_SIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /BLOCK_SIZE=n

    Output Save-Set Qualifier

    Specifies the output block size in bytes for data records in
    a BACKUP save set. You can specify a block size between 2048
    and 65,535 bytes. BACKUP may adjust this value according to the
    constraints of the BACKUP format.

    The default block size for magnetic tape is 8192 bytes; the
    default for disk is 32,256 bytes.