(info "(texinfo) @pagesizes") says: To make more elaborate changes, such as changing any of the page margins, you must define a new command in 'texinfo.tex' or 'texinfo.cnf'.
Mention a workaround: Before: $ perl -wle 'for(1..2){print "ABC " x 33, "\n";}'|makeinfo --plaintext ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC After: $ perl -wle 'for(1..2){print "ABC " x 33, "\n";}'|makeinfo --plaintext --fill-column=69|sed 's/^/ /' ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC