From Jeff Vance:

The CI's command line buffer is 511 bytes. The RUN command is parsed by the MYCOMMAND intrinsic, which has a 255 byte token limit, so, the info= string on the RUN command is limited to 255 bytes. However, "implied RUN" (like, XEQ progname "info") supports an info value up to as much as remains in the 511 byte CI buffer.