Changing default paper size for TeX Live

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

From the tlmgr documentation on paper:

paper

paper [a4|letter]
[xdvi|pdftex|dvips|dvipdfmx|context|psutils] paper [papersize|--list]
paper --json

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.

If --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 TLPAPER.

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 dvips. tlmgr does not support those extra settings.

This is described in the Quick install instructions:

The default is to configure the programs for the A4 paper size. To make the default be 8.5x11 letter-size paper, you can use the ‘o’ menu option before i(nstalling), or run tlmgr paper letter after installation (and after setting your PATH).