/sys$common/syshlp/helplib.hlb FORTRAN, Statements, OPEN, BLOCKSIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  BLOCKSIZE = bks

  bks  Is a numeric expression whose value specifies a
       number of bytes.

  For magnetic tape files, the value of "bks" specifies the physical
  record size in the range 18 to 32767 bytes.  The default value is
  2048 bytes.

  For sequential disk files, "bks" is rounded up to an integral
  number of 512-byte blocks and used to specify multiblock transfers.
  The number of blocks transferred can be 1 to 127, and defaults to
  the current count for the device at program run time.

  For indexed and relative files, "bks" is rounded up to an integral
  number of 512-byte blocks and used to specify the RMS bucket size.
  This must fall in the range 1 to 63 blocks, and defaults to the
  smallest value capable of holding one record.