From the command line, you can call
tlmgr with the
paper option to see what the default setting is, and change it if you supply an argument. So use
tlmgr paper letter
tlmgr documentation on
[xdvi|pdftex|dvips|dvipdfmx|context|psutils] paper [papersize|--list]
With no arguments (
tlmgr paper), shows the default paper size setting for all known programs.
With one argument (e.g.,
tlmgr paper a4), sets the default for all known programs to that paper size.
With a program given as the first argument and no paper size specified (e.g.,
tlmgr dvips paper), shows the default paper size for that program.
With a program given as the first argument and a paper size as the last argument (e.g.,
tlmgr dvips paper a4), set the default for that program to that paper size.
With a program given as the first argument and
--list given as the last argument (e.g.,
tlmgr dvips paper --list), shows all valid paper sizes for that program. The first size shown is the default.
--json is specified without other options, the paper setup is dumped in JSON format. For the format of JSON output see
tlpkg/doc/JSON-formats.txt, format definition
Incidentally, this syntax of having a specific program name before the
paper keyword is unusual. It is inherited from the longstanding
texconfig script, which supports other configuration settings for some programs, notably
tlmgr does not support those extra settings.