Library /sys$common/syshlp/ncphelp.hlb SET, CIRCUIT, MAXIMUM DATA number  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Applies only to X.25 PVCs and DLM circuits. Specifies the maximum
 packet size for the X.25 circuit. This value must be at least
 five bytes less than the MAXIMUM BLOCK value that you specify in
 the SET or DEFINE LINE command, and must be a power of two.
 Specify a value in the range 16 to 4096 bytes.

 By default, packet size takes the value specified by the PROFILE
 parameter of the SET or DEFINE MODULE X25-PROTOCOL command.  See
 the  Public Network Information manual for the network value of
 this parameter.