Library /sys$common/syshlp/ncphelp.hlb SET, MODULE, X25-PROTOCOL, KNOWN_DTES, DEFAULT DATA bytes  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Specifies the default packet size for X.25 virtual circuits. This
 value must be at least 5 bytes less than the MAXIMUM BLOCK value
 you  specify in the SET LINE command and must be a power of 2.
 Specify a  value in the range 16 to 4096 bytes.  By default
 packet size takes the network value specified by the  PROFILE
 parameter.  See the Public Network Information manual for the
 network  value of this parameter.