The  ENUMERATION_SIZE  attribute  controls  the  allocation   of
  unpacked enumerated data types and Boolean data types, which are
  considered to be an enumerated  type  containing  two  elements.
  Note that specifying this attribute overrides any value that you
  previously specified with the /ENUMERATION_SIZE qualifier.

  Syntax:

      [ENUMERATION_SIZE(keyword)]

  You can specify the following keywords:

  Keyword               Action                     Default Information
  -------       ----------------------------       -------------------
  BYTE          Allocates unpacked enumerated      Default on OpenVMS VAX
                types with fewer than 255          systems.
                elements in a 8-bit byte.
                Otherwise, the enumerated types
                are allocated in a 16-bit word.

  LONG          Allocates unpacked enumerated      Default on OpenVMS Alpha
                types in a 32-bit longword.        and OpenVMS I64 systems.