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

 Specifies the length (in bytes) of the blocks to be sent
 during loopback testing. The length must be a decimal
 integer in the range of 1 through 4096. If the parameter is
 omitted, a block length of 4096 bytes is used.