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

 Applies only to X.25 lines.  Specifies the maximum size in bytes of
 the frame for the line.  This value must be at least 5 bytes larger
 than the maximum packet size you specify in SET CIRCUIT (for PVCs and
 DLM circuits) or SET MODULE X25-PROTOCOL (for SVCs) command.  Specify
 a value in the range 21 to 4103.

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