以前は
% sudo tl-paper set all a4
/usr/bin/tl-paper: setting paper size for dvipdfmx to a4.
/usr/bin/tl-paper: setting paper size for dvips to a4.
/usr/bin/tl-paper: setting paper size for pdftex to a4.
/usr/bin/tl-paper: setting paper size for xdvi to a4.で defailt の paper size を letter から a4 に変更できたはずなのだが，なぜか効かない．上記でたとえば pdftex (pdflatex)なら /var/lib/texmf/tex/generic/config/pdftexconfig.tex が以下のように変更はされている．
--- pdftexconfig.tex.orig 2020-08-07 20:48:23.642640762 +0900
+++ pdftexconfig.tex 2020-08-07 23:00:43.181888255 +0900
@@ -6,8 +6,8 @@
\pdfoutput = 1
% Paper size: dimensions given in absolute terms
-\pdfpageheight = 11 true in
-\pdfpagewidth = 8.5 true in
+\pdfpageheight = 297 true mm
+\pdfpagewidth = 210 true mm
% Enable PDF 1.5 output and thus more compression
\pdfminorversion = 5どうやら /var/lib/texmf/web2c/pdftex/pdflatex.fmt を作り直す必要があるらしい．以下のようにdpkg-reconfigure texlive-baseするか，そこからトリガされているtex-commonをdpkg-reconfigureしてやればいいらしい．
% sudo dpkg-reconfigure texlive-base
mktexlsr: Updating /var/lib/texmf/ls-R-TEXLIVEDIST...
mktexlsr: Updating /var/lib/texmf/ls-R-TEXMFMAIN...
mktexlsr: Updating /var/lib/texmf/ls-R...
mktexlsr: Done.
tl-paper: setting paper size for dvips to a4: /var/lib/texmf/dvips/config/config-paper.ps
tl-paper: setting paper size for dvipdfmx to a4: /var/lib/texmf/dvipdfmx/dvipdfmx-paper.cfg
tl-paper: setting paper size for xdvi to a4: /var/lib/texmf/xdvi/XDvi-paper
tl-paper: setting paper size for pdftex to a4: /var/lib/texmf/tex/generic/config/pdftexconfig.tex
Processing triggers for tex-common (6.15) ...
Running updmap-sys. This may take some time... done.
Running mktexlsr /var/lib/texmf ... done.
Building format(s) --all.
This may take some time... done.