Re: Custom page size

[karl]

    Could you please advise me how should I modify texinfo.tex to
    accomplish this task?

You don't need to modify texinfo.tex (unless you want to).  Instead,
create a definition in texinfo.cnf (sitewide or in your document's
directory or whatever), following the example of, say @afourpaper, and
changing the numbers to suit.

@tex
\gdef\afourpaper{{\globaldefs = 1
  \setleading{12pt}%
  \parskip = 3pt plus 2pt minus 1pt
  %
  
\internalpagesizes{53\baselineskip}{160mm}{\voffset}{4mm}{\bindingoffset}{44pt}%
  %
  \tolerance = 700
  \hfuzz = 1pt
}}
@end tex

See texinfo.tex for a description of the arguments to
\internalpagesizes, so you know what to put in.  (The first arg is the
height, the next the width.)