You can format a document for printing on ISO 216 A4 paper size with
the @afourpaper command. Write the command on a line by
itself near the beginning of the Texinfo file, before the title page.
You may or may not prefer the formatting that results from the command
@afourlatex. There’s also @afourwide for A4 paper in
wide format, @afivepaper for A5 paper, and @bsixpaper for
B6 paper.
See Format with texi2dvi or texi2pdf
and Preparing for TeX, for other ways to format for different
paper sizes that do not require changing the source file.